The dif with LIPO (rather than nimh) is they can output a much higher continuous current. This will turn the motor faster and can put stress on stock internals, but only really if you use an 11v lipo will it potentially damage it. Some more fragile or high-speed/rof guns might take a bit of wear from a 7v lipo, but you can always get one with a lower C rating if you're worried (i use 20c 7v lipo on my TM high-cycle)
Basically to use a lipo all you need is to plug one in, you might have to change the plug depending on what battery connection you've got. With LIPO it's recommended you install a MOSFET, a little electronics bit designed to bypass the trigger and stop the contacts burning out eventually from the higher current compared to NIMH. There's lots available and wiring diagrams floating around. If you're not comfortable with soldering then perhaps take it to an airsoft store to do for you.
In terms of the internals, as far as a gear box goes you kinda have to upgrade it all, or you're left with a weak link in the chain and that'll be what takes the brunt and breaks if your motor is kicking out more force than the gun was designed for.
In summary all you need is; 7.4v lipo battery of appropriate size, mosfet, perhaps dif battery connectors, and a gun that's internals are made of metal not plastic.