I was wondering the same thing!
I personally couldnt tell a majotr difference between my R100 and Paul's bike--
WHen I say I couldnt tell a difference, I mean of course I could, they were amost Entirely dis-similar but i mean that i did not miss 130cc by a long shot.
The engine pulled long and hard, for as long as my courage held out.
And it seems shortsighted to simply put up with the extra two inches of width and whatever extra weight came with the R100 motor, even if it is easier.
Hows this for an idea:
SInce the short width of the R65 stems from the short con-rods, and one is required to shave the cylinder bases down anyway, is it feasible to simply put short con-rods in an R100? With short headed pistons, (JP's?) one should be able toget he same volume out of the jug... I doubt it, I bet the conrod bores are the wrong size, it would be a custom con-rod deal. Maybe thats ok?
Then there's making the pushrods do right. Use R65 pushrods?
Paul is not unenlighted to solve for the lightest combination possible. Everything we talked about, he was stressing lightness.