If you look at your bike and look how much room there is between the frame and the engine backplate,that will give you some idea of what you can fit in there,that's why I suggested an OS pull start engine with the plastic pull start removed.

A picture of a TT.12 with the TT pull start backplate fitted.The backplate was turned to remove the surplus pull start lugs.
There has been a Rossi.12 fitted since this picture was taken.


Homemade Rossi backplate conversion.


Allan