Delrin bolt of some sort (Eclipse DART, Voodoo etc).
Eclipse valve kit.
LPR.
New hammer and shaft.
Lo-rise.
I-Frame.
You can upgrade Impys all day long and spend a fortune on them
Best bet is to probably work out why you want it upgraded if indeed you need to upgrade at all. Do you want better effieciency, speed etc etc
Play with it before you think about upgrading fella, don't think you have to upgrade it, it may run just how you want as you get it so there may be no real need to spend a load more £££
Dave