you can fit an oversized piston but you would need to get the barrel bored out to accept the extra width, go on ebay, there are big bore kits for about £45 on there, it is basically a 60cc piston and a whole new barrel which replaces your old one, i had one and it was great, used to get 50mph+ from it but didnt jet my carb and it chewed itself up.
you will need to rejet your carb, i was originally going up from a 99 to a 115. to get the best use out of it i suggest you also look at getting an aftermarket exhaust and a pod style (K&N) air filter, both of these would require re-jetting (mine already had a filter and exhaust together and came jetted from a 88 to a 99) and are cheap and easy to do
if you are wanting more speed then perhaps change your gearing?, the largest front sprocket available i think is a 14t and the smallest rear is a 44t which will give a noticeable increase in top speed at the expense of acceleration.
ive tried every trick in the book to get more speed out of my DT and the exhaust is probably one of the easiest ones to do and give a surprising amount of extra power, it lets the engine breathe a lot better and when used with a K&N air filter causes less restriction in the engine,