Using heads and/or cylinders off a 70.6mm stroke motor won't work on an R45/R65 case. The difference in stroke makes developing a decent compression ratio difficult. If you are going to use the R45 cases and crank I suggest using the stock heads with valves from an R65. Bore to 1mm oversize; you can get pistons from a number of aftermarket sources.
On the other hand, you could really cheat, taking the motor out to 750cc. As long as you don't look significantly faster than the other bikes, or win too often, no one should ask for a tear down