I woulda said screw a plug into where the muffler goes -- half inch pipe thread. If he's mechanically minded, pull the exhaust valve out and grind the end way down, so that the camshaft doesn't lift it.
I'd leave the camshaft in. I don't know all the details, but the Briggs guys did a LOT of work to make things balance correctly.
JC Whitney did used to sell a hose that threads into a spark plug hole, and provides air for emergency flat repair, and so on. Dunno if they still have them. Wonder what's the next largest pipe thread? Ah, well. Something to play with.
Be sure to keep the crankcase at the proper level of oil, you'll still need the splash lubrication of the piston.