Is it gas or diesel
If gas could be simple as spark plug leads or points
If diesel could be more expensive
Could also be a valve ??
In terms of transmission, could simply be the detent springs that hold the shifter rods in place
Would advise taking off gearbox cover. and looking in gearbox, would have to drain oil to get a good loook at the gears, to ensure no very worn or broken ones.
They are good tractors