Well it appears i now have the knowledge to answer my own question.......
Yes timmy you should be able to drop in any small block .15 or .18 into the rc10 however new era do a .21 mount if you want to go bigger, Not sure about your roto-start question but i have seen roto-start for .12 - .18 engines advertised, IT appears there are 2 types of starting mechanism one where the one-way bearing is just under the pullstart and slots onto a starter shaft coming from the engine (thru the backplate) and the other where under the pullstart there is just a smooth bolt (no threads) which goers into the backplate and the one-way bearing is inside the engine which this bolt goes into, I dont know what the names of these 2 designs are.
ANYONE?
But i know that the second design (beaaring inside the engine) appears to be older and now obsolete, As my friend Dan has been informed they no longer make the (AE)pullstart for his RC10GT and the other one doesn't fit cos one-way bearings are bigger than the hex part (top) of the bolt style he needs and so the hexagonal hollow on the puyllstart is too big
ANYONE know where he can get this, I will get the part number from him if needed.