Im a firm believer that a simple 170 -200 bhp n/a engine in any westfield/caterham/indy etc etc will be as much road performance as most folks can cope with.

(Porkie excluded! )

And indeed a similar car on track (driver permitting) will be very very hard to catch and pass.

The only bike engined ones ive seen/spoken to on track days, have all said the lack of torque is an issue, yes fine when high in the rev range but no where near the torque of a simple 4 cylinder car engine.

You pays your money you takes your choice i suppose?