I've never done it as ours is already electric start, but I think you have to get a torque tube section (between the transmission and engine block)that has the opening for the starter. Split the case on your tractor, pull the main shaft and whatever else and swap the torque tubes out. Of course you'll also need a starter and the linkages, battery and switch and wires. Assuming that the crank start/magneto tractors don't have a generator and lights--then you'd also want to add generator or alternator and lights.
Hope that helps.