Sorry, I've been real busy, and away from my Mac for a week. Going from a combustion chamber size of 43cc to 58cc would decrease the compression by nearly 2 full points, but I wouldn't advise it. The rest of his combo is designed to work with the higher compression. For example, he's either got a high lift cam with a very short duration, or fly-cut pistons (or both). Does he know which? It will effect your performance. Bottom line: the carb will be too big, the cam won't work as well, the heads would have to be changed, etc. Why? Just build your own. That way you'll know what it's got inside. His probably needs new rings and bearings at the very least, and really wouldn't be worth the effort, IMO. If you want a race car that isn't going to be driven on the street, or at least not without some 110 octane fuel, then trade him. Otherwise, build what you want yourself.
You asked.
Take care,
-Chris