My only point of reference here is from Matsu Matsuzawa, where he stroked his 125 to 185cc:
1985 Matsu talks
should help on the low end anyway.
If you could remove 5 cylinders that would help too
There might be something about piston velocity too.
http://ecomodder.com/forum/showthrea...peed-1477.html (Or it is just a symptom)