Thanks guys for all the great info, Kevin this is Jeff's engine. The 4mm drive shaft is a good idea but the bigger issue is the plastic gears in the gear box. Being they are plastic even if i replace them with new one's they can easily strip again. Like Chuck and Pete said above it's not easy to find Metal or brass gears. Pete is on the hunt for metal or brass gears, let's hope he can find them so we can repair these engine's the right way. Keep your fingers crossed.
Thanks,
Alex