Hi Mate,
You can fit a larger motor in, although you may have to change a few things depending on the clearance from your front wheel to the engines head.
I have a china 50 with a 110cc loncin semi auto motor in it. It has longer forks to allow the front wheel to clear the head. It has a huge amount of clearance though and it may have even fit with the stock forks.
Lots and lots of fun though.
In a nutshell. Yes you can, without too much bother.
Logic.