Sounds like a rocker arm or tappet on one of the valves. If it can be repaired without removing the cylinder head, it shouldn't be too bad.

A shop would take the valve cover off, protect the engine bay from oil splashing all over, and start the engine to identify exactly which valve it is.

If it's a broken valve spring, bent valve, etc, then the head probably has to come off for the fix. That would not be good.