I think the stroke may be different, possibly a different crankshaft, or a shorter/longer connecting rod.
in my opinion, the cost and all the trouble doing it would not justify the effort.
48 cc.'s and 66 c.c. make about the same power and you probably wouldn't even notice any difference. have a 48 c.c. that will walk away from my friends 80 c.c. any day of the week. same style bikes, same size sprockets, same basic set up between the 2 bikes except i have an exp. chamber, and he weighs about 50 lbs more than me.
even if we switch bikes, my bike will still walk away from his.