If you wont see over 200kmh, its the GEARING that will make the most difference, powerband has a lesser effect.
Twins are a waste of time personally.
What power you want? Personally id gear the thing down to it would top out at 7.5k in 5th at just over 200kmh, get it as light as poss, and go for about 350bhp or so, but with a fairly low comp so can run a small turbo at very high boost, for maximum response and maximum torque.
Something like a EscCos T34 with the GrpA .55 ex housing would be lovely i reckon, loves 2bar boost thru it too.
If you wanted something a bit more flash then i dunno, GT2871 or summat.
and decent tyres and brakes too, as sounds like a short course with lots of tight corners to punch out of fast with the aformentioned torque and response the engine and gearing would give you.
DISCLAIMER- Thats jus me tho, and everyone here seems to have other ideas, that someone told them once, lol.