Rounded Allen key bolts - drill the heads off, remove engine and then twist the remains of the screw out with some pliars. Then replace with new bolts.
Metal replacment parts - waste of money, I spend hundreds on alloy bits on my old ts4n, looked pretty but made no difference whatsoever to handling or speed.
Changing gears - if you've got the 2 speed then you just remove the e clips holding the gears onto the layshaft and the 2 speed assembly will come out, then there's 4 small screws holding each plastic spur gear onto the the transmission..
Cleaning without stripping down - cotton bud and bog roll usually works well