10.5 Briggs Not Starting

"I'm thinking I might have an intake valve stuck open. Engine seems to have good compression,"

Don't think you would find good compression or any if Intake Valve was stuck open. Since this is a pull start engine I am guessing it would not have a solenoid in the bottom of the carb? Have you tried removing the air cleaner and dribbling some gas from a squirt can or plastic bottle directly into the carb throat to see if it will fire? IF not, I would check the flywheel key given you have good spark and fuel. Very unusual for a key to shear on a belt driven mower but maybe it was not properly torqued to start with.

Walt Conner

"I checked compression and mine had none! The valve seat on the intake valve had popped out and was holding the valve open.
To check this, remove the spark plug, place your thumb over the hole and slowly pull the start rope. "

"Engine seems to have good compression, " (from original post)

Walt Conner