identify this broken off boss that was jamming the clutch fork

Professor Fate said
You need that boss- it not only locates the starter in the bellhousing, but it supports the end of the Bendix shaft.
If you can't weld it back on, contact Powermaster and see if they will sell you a new nose piece.
If it's based on the Mopar mini starter, you might be able to use something like this:
NOSE CONE FRAME for MINI STARTER MOPAR DODGE PLYMOUTH (HOUSING only for repair) | eBay

It does not support the end of the bendix shaft in this starter.
The hole would be machined or at least have witness marks if the shaft entered it, and it has neither.
Nonetheless if the nose cone you linked will fit the Powermaster starter I'll install it, thanks.