I believe a touring bike motor would make a great powertrain for a BEC Locost. Some are shaft drive, all are tuned more for the power range a light car will benefit from, and they are likely to cost a LOT less than the sport bike engines. Not much of any demand for them. Yet.
They are also less likely to need expensive rebuilds or constant tuning.
If you want a track car then the sport bike engine might be more desireable, but for all around fun driving I say go for a touring bike engine.
Honda makes a fantastic V-4 touring bike that I can't think of the name of now, or if it is still in production, but I remember reading tests of it some years ago and thinking it would make a neat motor in a car.
The new Concourse 1400 is fantastic.
The Honda VTX cruiser has a dry sump stock which is a real plus.