[Gaz64]

I've had that before with one engine that I rescued.

I would wind the engine over to place the cylinder at TDC, then remove the rockers, remove the spring and retainer etc.

If all the valve stem tips are still the same height, and the valve stem is ok, then leakdown test the cylinder, if it is good then continue with rest of repair.

More than likely, the head will need to come off.

If one valve has done this, what about the other 15?

There are many clearances to check in OHV valvetrain.