Bob, if the inner ‘cable’ of the tach cable is broken, and you can’t find a replacement, check out a speedometer cable repair kit.
A long time ago, I repaired a speedometer cable with one of these kits. As I recall, the square end of the cable was OK, but there wasn’t a proper matching ‘other’ end in the kit. I reused the original ‘other’ end by soldering it to the end of the now shortened kit’s cable. Brazing or epoxy could/should work also???
Id
P.S.
"tach" doesn't pass the smell test of the speel chekker.