I came across this looking for a 300cc cylinder.
Didn't find it (yet) But this was quite interesting too.
Carter GTR 250/300 Owners Manual.

Jack

So is a 250 just a pistion and head away from being a 300 ? I keep calling carters and nobody seems to know that awnser.They said you be better off selling your 250 and buying a 300.I talked to a dealer who said its the same motor,just 50cc larger.So how do we get that extra 50cc with out buying a new 300,parts and tech department arent going to spill the beans!!!!!

Hi Gocartus,
Actually I was wondering the same thing.
Carter importer for the Netherlands wasn't any help on this query. Must be a company policy or something. I think they all just push large boxes around and don't know much about what's in them.

Next I asked my parts dealer who trades in SYM ATV's in Germany.
He didn't know either because he doesn't do tuning on them.
But he contacted SYM Taiwan directly and got back to me.
SYM Taiwan said that both halves of the crankcase are different for the 300.

Exactly what is different they didn't say. I suspect the crankcasing is just a bit wider at the neck to take the larger bore (something that i could easily mod myself). But on the other hand maybe they fitted larger bearings too. I really don't know at this time.

Maybe there's a 300 owner or dealer on here that can help us.
answering that question.

Jack