I've had a few fast bikes over the years, including a turbo'd 1402cc Suzuki.

A large capacity turbo'd bike engine would be fantastic in a lightweight shell. Critical that the car is extremely light weight, and that the engine is largest capacity possible.

Bike engines are traditionally targetted at high end power which is great, but in many cases they lack low-down torque. On top of this, to suit a heavier shell you need to re-tune the engine and it will make less power & torque than standard unless you turbo it or complete additional modifications.

That said, you can have some BRILLIANT results with this kind of thing.

Personally I'd look at a GSX1400, GSX1300R, ZX12R or ZX14 engine.