Like what? Do you wish to modify its look? or do wish to tinker with the engine and gear systems etc to increase the power output? Also mention the specifications of you Bike. Till then get a bigger rear tyre. I have a 150/80 R15
on my Royal Enfield. It could not support a bigger tyre. Change the silencer. Better still remove it and install a hollow pipe in the shape of a silencer. The exhaust gases will have no resisting leaving the exhaust system and will increase the acceleration of you bike. All these will be at the expense of mileage. For some visual alterations you can get a new tank, handles, make some alterations in the design to make it more lighter and aerodynamic which will in turn increase the acceleration by a small amount and also your mileage. Tell more about your Bike and i will give you further options.

I hope Shameem posts an update on this, could be a good thread...