Second major issue for a BEC* build is that unless the Bike engine is Dry-Sump from the factory, or converted, oiling is a problem.
Bikes lean so the oil stays in the sump, cars have have too much side thrust so the oil climbs the side of the sump and starves the pump.
They can be a great conversion. Fairly common in Locost and Spridget builds.
I've thought of doing one in a Spitfire but really do have more projects already than makes sense.
Scratch built car I was very intent on doing as a BEC is now getting a Suzuki 13B twin cam instead as it will be MUCH simpler and avoids the oiling issue/expense.
The 13B is an amazing little engine with a lot of similarity to the Renault TS.
Sadly a bit hard to find in the U.S., but maybe a little easier in Canada?
*BEC = Bike Engine Car.