I’m not making a documentary, I’m making a drama,” Academy Award-winning director Ron Howard tells me as we walk back from lunch at Crystal Palace where he is directing the new film Rush on Niki Lauda and James Hunt.

It’s a surreal experience talking to Richie Cunningham from Happy Days, and his enthusiasm for his latest project – due out in cinemas in February/March 2013 – is infectious. Many Formula 1 aficionados have already started picking holes in various scenes having seen photos of them on the internet, and some have asked how Ron Howard can make a film about a sport he has never really been a fan of. “Well,” he says to a Sky reporter while I’m waiting for my turn, “I’ve never been to the Moon and I made Apollo 13.”

Ron Howard may not be a lifelong fan of F1, but he has surrounded himself with people who know the sport and know what looks right. The plan is to tell the story of Hunt and Lauda’s relationship between 1973 and ’76, but Howard doesn’t want to get caught up on details. Yes – it will be based on fact, but there will be drama added. That’s no bad thing so long as it doesn’t resemble Driven, the dreadful Sylvester Stallone film based (very) loosely on Champ Cars. Howard is adamant this won’t be the case: “I know what would annoy me if someone made a film about the sports that I love,” he tells me.

One of the men working behind the scenes is Darren Stone of Mirage Engineering. He has been tasked with building some of the replica F1 cars for the film, which he is well placed to do, as Mirage Engineering’s day job is to run customer cars in the FIA Historic Formula 1 Championship. “I guess that’s why we were approached [in October last year],” he says.

“Special effects companies probably could have built the replica cars, but they haven’t got the knowledge” Darren Stone, Mirage Engineering

“Special effects companies probably could have built the replica cars, but they haven’t got the knowledge. The fact that this car would have revved higher than that one, this would have been a V12, which would have sounded different to this one…”

Many of the cars that raced in period are still in running order today, but the cost of using those in filming, and getting proud owners to allow precision drivers – ones used to working with cameras – behind the wheel was a step too far. The decision was made to build replicas and use these alongside the available historic cars for the close-ups. Rob Austin – of British Touring Car fame – was given the job of building the Ferrari 312Ts, while WDK Motorsport took on the McLaren M23s and the Hesketh 308B and 308D. The BRM that Lauda used in 1973, though, was given to Mirage Engineering.

However, it wasn’t just a case of copying the sole surviving car – which currently rests in the Donington Collection – because the powers-that-be wanted Mirage’s pair of BRMs to have interchangeable bodywork so they could also be a Ligier JS5, a Shadow DN5 or a March 761. The filmmakers could not source the correct Ligier at all, so rather than erase it from history they will use the replica.

“When we first got the brief,” says Stone at his base in Norfolk, with a cup of tea in hand, “the cars just had to trundle up and down. By the time we were building them [a task for which they had only three months] they were saying ‘we need to do some standing starts’. Luckily we had made the right move by using a Rover V8, but we did have to uprate the clutch. In the end we went with Sierra Cosworth driveshafts so they can wheelspin them all day if they want.”

I can imagine some of you wincing as you read ‘Rover V8’. The BRM P160 had a V12. The sound, though, will be dubbed on later, and what Mirage Engineering needed to build was something that looked right.

After a trip to the Donington Collection with a tape measure (I’m not joking) and a camera, they had a starting point. “We did do a scan, but the trouble was using all that information and interpreting it in the short time frame we had. We didn’t have time to do drawings so we just went with ‘this looks right, make it’. We trawled through Google and looked at every BRM picture we could find and we also found some scale drawings from a modelling site, which were really helpful. The geometry of the suspension (left) is spot on, for example, but we needed to use as many stock parts as possible so that it would be reliable. The radiator? That’s off a BMW 535i – it just happened to fit…

“The guys at WDK and Rob Austin’s place have managed to base their cars on existing chassis, but we realised very early on that we were going to have to build one from scratch because the thing that concerned me was that the BRM’s rear end is so exposed. They didn’t have the bodywork to cover all that like the Ferrari [312T] and so we had to do something a little more accurate.”

A spaceframe chassis was soon made and the task of fitting different bodywork – made out of MDF and fibreglass – onto the same car was refined. “With a team of four guys we can change the BRMs into a Ligier or a Shadow in half a day,” says Stone, “but one of the biggest problems was that they needed silencers on the exhausts so that they could run on ‘quiet days’ at circuits. We’ve got big silencers in the sidepods and with the other cars – like the March – we couldn’t get the right sized bodywork to fit. We had to stretch the sidepods a bit! Also, the BRM is a shorter chassis than the Ligier so when we made up that huge air box off the JS5 it looked ridiculous! We had to shave it down and it still looks too big. They must have been on drugs when they made it – it’s madness.”

The attention to detail is impressive, and when you see the cars from a distance they look just as they should. Of course, the cars were changed from race to race, but as Stone points out they just had to choose a moment in time and build that particular version. The BRM P160E in the Donington Collection was chosen as the ‘moment in time’ for the replica.

“Even with all the work that’s gone into these it’s a lot cheaper than running the real thing. I suppose in the scheme of things it’s quite big money [the pair of BRMs came in at just over £100,000], but that was starting from scratch and building two working cars in a short timescale. There are lots of things that I would have liked to do better, but it’s all down to time.” The team at Mirage has been working flat out and trainee mechanic John Denmark pointed out “there are a surprising number of hours in the day when you work for 23 of them”.

As I’m leaving talk turns to the other possible motor racing films that have been mentioned of late, such as a touted Dick Seaman movie. A replica Auto Union – surely that would be a nightmare to do?

“Oh I’d love to make one of those! I’m already thinking about which engine I’d use, what chassis I would base it on. I can see a Special Vehicles arm of Mirage Engineering being born…”