First of all you need to diagnose the engine properly. That obviously hasn't been done. Adjust the valves, do a wet and dry compression test. Maybe do an old time oscilloscope ignition evaluation. Check the carbs for proper adjustment. Then you can decide if the engine needs to be opened up further. If you have a burned valve, then replace THAT VALVE only. Grind the rest. If there is no exhaust valve seat recession, then grind the seats and put it back together. If you have recession, install seats AND new valve springs. Check the installed height and spring pressure on the seat. If your machine shop can't do that, then they aren't an automotive machine shop.
Conjecture isn't diagnosis. It's just talking into thin air. Diagnose the engine and fix what is wrong, and nothing more.