If you're going this way, you "may" want to sell the 69mm crank. IMHO, those heads are too big and will fall on their face with a near-stock displacement.
I'd get a case already clearanced(and don't forget the camshaft itself!) for a stroker crank.
A 78mm or better(82, 84, or even 86) crank will help you use those heads for proper breathing.
64cc heads are old-school low-compression. Some milling machine work to make the chambers smaller is most likely in order. Mine are 52cc, my static CR is 10.2:1. But admittedly I'm at the sharp end of the stick.
The important thing to remember is that EVERY part of the engine has to be sized to work together. Dynamic compression is also a big factor. Cam/ports/rocker are all related and important.