Nick: I'm not certain on this, but I was talking about this to someone the other day, he was showing me some 3-wheeled motorcycle-powered kit "cars" in from America and the UK. Anyway, the problem is the engine has to meet emissions of the engine itself (which I'm sure it won't have a problem doing), and motorcycle engines aren't tested for emissions the same as car engines are. So, in order to get a car like this engineered, you'll not only need to get all the mechanicals engineered (which would likely be a nightmare), you'll also have to get the engine tested for emissions under "car regulations". That, as you can guess, would cost a lot of money.
Some Locost people down south had to get a modified silvertop 4AGE tested awhile ago (to comply with current emissions, since the silvertop is an older engine and they're putting it into a "new" car), and I remember it taking a lot of time, and ending up costing a lot of money. But now they sell the modified computer etc to other Locost builders who want to use silvertop 4AGE's in their cars, so they're slowly getting the money back...
Or something along those lines.
Just the same, to get my car engineered it had to comply with regulations of a 1991 4AGE, not a 1975 Corolla...