Dunno about the gas TM MP7, the electric does have few upgrade options available but it is a bit tricky.
Laylax (Nineball) have made aftermarket/upgrade bits for a majority of the gearbox and a couple tightbore barrels, but these aren't easily found from UK retailers, didn't find one that had them all anyways, Eagle6 has most of them.
The exception is the gears and motor, no-one seems to have made replacement gears with a dif ratio for increased ROF, and no-one seems to have made a replacement motor at all, can't even find an original one so if that breaks then your fuked.
Converting to lipo is a popular one as the standard MP7 battery doesn't last too long, can either rewire to a PEQ, or chop the connector off a standard MP7 battery and attach it to a lipo small enough. I didn't have a stock battery so bought a Laylax MP7 lipo adapter as that'd done half the job for me and same price as a stock battery anyways.
Also it's a tricky little bitch of a gearbox to tinker in, took me several attempts to get it perfect, end result is about 310FPS and about 17-19RPS compared to stock of 250FPS and about 15RPS