Dave the guy who does all of my paintwork, has an Aprilia RS250, which has been fitted with a motor based on the 350 powervalve motor, but with new crank, cylinders, heads, pistons, FCR39 etc, 500cc and 120bhp. He's been building this bike for the last few years, with radial braked front end, lighter wheels, race plastics etc. This bike should weigh in at around 140kgs wet.
The motor has been built by an Aussie mob called the twostroke tuning shop or something very similar. They designed their own cranks, cylinders, pistons, heads, powervalves, ignition system etc etc. If you look on youtube there are some vids of the 500cc motor in the RS250 chassis.
Personally I'd be looking at grafting two RM250 cylinders onto an RGV 250 bottom end to make a V twin 500cc stroker, and slotting that into something like a 125cc or a lightweight trellis frame.
Of course if money was no object I would be building a V twin twostroke 1000cc motor from scratch based on something like a pair of CR500 cylinders or similar, I would make it a modular construction, to enable two of them to be grafted together into a 2 litre V4 stinkwheel.
Failing that using a 275bhp V4 stroker motor from a snowmobile with a dry clutch and bike gearbox would suffice.
I only weigh 50kgs, so I can happily sacrifice a few kgs of extra weight on the bike as the total package weight will still be less than 200kgs wet with rider. (My current bike is 220kgs wet with rider and 144bhp 70lb/ft of torque) I can get the weight lower if I go for a good shit before putting my leathers on!!!