In Antique Power magazine there is an ad for tach repair. The name is Metro Speedometer LLC. # is 1 (816) 478 0116. They are in MO., and I know nothing about this outfit other than seeing their ad.
When I purchased my tractor it had a seized up cable that apparantly wore down the gear drive in the distributor. I had to replace the distributor and get a new cable. Hooked all that up and was fortunate that my tach still worked.
Good luck with yours.