I suppose that the valve could have seized, it is hard to tell.
I am fairly confident that it did not, head was rebuilt by a very good machinist with all new valves/guides.
but the crank sprocket failure was clearly secondary, there was no back and forth wear, just a clean break in one direction, as if the belt stopped moving suddenly.
I spoke with my grandfather(longtime engineer/machinist with a background in metallurgy) about it, and he was sure that the metal taper was a poor design, which would only work properly if the taper on the gear was slightly tighter than the camshaft, which would only work once.
I am not sure what happened, but I am hedging my bets to try and keep whatever it was from happening again