I used to race in supermono class so know a bit about big singles. First i had a honda ft500 engine in a tz250h chassis, wonderful bike, 3rd in series 1988, couple of lap records and many wins. You can get about 60 bhp out of these engines but they can be fragile. Mine were between 45 and 55 bhp. Also had a 600cc rotax in a tigraft chassis, also a lovely bike. You can tune these engines upto 80 bhp (depends on your wallet) also the chain is on the right hand side. Would be ok on mc18 but not 21 or 28 but would be nice in a rs250 chassis with singe sided swingarm?. Also had a susuki dr800 in a tigraft chassis, did'nt tune it too high. Was very quick and i could paddle start it! Proberly the best engine for your project would be a yamaha xt600/660/680. Very tuneable and easy to get parts for and still competetive in racing (have got a xt660 in a works mz (tigraft) chassis for sale £1200). Hope some of this may be of some use.