Starter drive problem in UTB 650 universal tractor

Hello I have starter drive not engaging on a UTB 650 tractor,Its a huge looking starter to turn 4 cyl diesel probably the original starter,I have taken it to a electrical rebuild shop and the guy cant seem to find a replacement so he managed to take the drive apart and clean the rollers and ramps but it takes many tries to get it to turn the engine.I am asking If anybody here is familiar with this tractor enough to give a replacement number for a new drive or complete starter.Thanks

Starter drive problem in UTB 650 universal tractor