hey dude, before you get an expansion chamber, take your carb apart and clean it. also, when you take the carb off, look in the intake. the intake should be the same size or larger than the throat of the carb the whole way through. if there's plastic, or even a matal piece cutting it down get it out, no matter how you have to.
also, if the rings are different, that could seriously affect your compression. that's a ridiculous way to change it, but maybe they actually did that.
you can also look at your timing, try advancing it a little bit. that can gain you a little bit in some situations.
usually if you find every variation in parts for fast vs slow bikes, you can replace the slow stuff with the fast stuff.
is the exhaust the same?
is the needle position in the carb the same? are the jets the same?
thinking of hondas, does it have the same cylinder for sure? more/less gaskets in any place?