Ok, newbie-ish mistake on my part; yes cg engines are the air-cooled Honda designed in the 1970's or 60' engines found on many a Chinese bike today. The newer, larger engines are based on newer, liquid-cooled engines.

A few years ago, member 'Wild Alaskan' of chinariders.net succesfully mated A Lifan GY 5 enduro chassis to a XR400 engine. Perhaps those are closer to what you seek, and are somewhat plentiful in the US? Come to think of it, another guy on youtube did the same thing to his bike which happened to also be a GY 5 Lifan, documented on youtube. That guy's name on youtube is Sheridan Joyce. His 400 was out of an ATV.

Ok, tired tonight-I think 'wildalaskan' and 'Sheridan Joyce' are one and the same.