Hey there Mini09,
I was looking into this a while back, as an option. still might do it one day
Both these kits use the Yamaha R1 (998cc) engine from the late 1999 -> Early 2003, after this the motor changes it size and doesn't fit under the bonnet. Both kits advise adding disc brake system as you are putting a 140-160 hp engine in the place of one giving 37 hp.
Try this kit, come with new sub-frame (advantage is a mechanical reverse driven from bike engine via special box) bike engine faces rearwards, carbs to the front, requires plumbing radiator to the other side.
http://www.pro-motive.co.ukTry this kit as it has no or little modification to mini sub-frame, the kit bolt on bolt off (reverse is optional via a converted electric starter motor) bike engine faces forwards exhaust forward, allowing use of existing radiator.
http://www.lynxae.co.ukZ-Cars is supposed to be getting back up and running but this has yet to be seen.
You could also try making your own Aussie custom sub-frame like the V-Tec and Suzmini guys!
Hope this helps.