I bought the replacement voltage reg for my TO-35 at autozone, reg was made in usa by borg warner, cost about $60 but was the best one I found local.
someday I would like to install a alternator on my tractor with a built in reg. but my old tractor had the gear box on the back of the generator that runs the tach.
so I do not know if I could replace the tach with an electronic tach that would run off some other electrical source.
generators, voltage reg and gear driven tach are old technology.