Modding Bikes? Sprockets? Big Bores Etc

Hey Chaps So im moving onto an Enduro style bike as all my college mates Go off-roading and im missing out on the fun also I would like some gears

Im Strung Between Two Bikes right now the Suzuki Ts50 and the Yamaha Dt50 Does anyone Know of any other Enduro Style Bikes?

The Main thing i wanted to know is How Modable are they? Can you 172 them?

My Mate Put a 70cc kit in his TS That used to do 35 now it does 60-65 All he changed was the Jet in his Carb.

Just Need some Info Really on moding Geared bikes what needs to be taken into consideration
Thanks