Hi Morgan,
Thanks, glad you like my air starter. It is about as simple as it gets.
I had similar thoughts for autostart (air ram to press wheel/air motor against flywheel, with spring return).
Another thought: When the engine starts, the air motor speeds up as it is now driven by the flywheel instead of vice versa. This might be detectable via a pressure switch/sensor, though an IR LED/phototransistor through the flywheel spokes could also be used to sence RPMs to shut down the starter air.
Air rams and solenoid valve are not "out of sight" price wise, either. Grainger has a nice selection.
Auto start is something I probably won't do this year, though I do want remote shutdown to same a trip to the genshed on shut down.
Bruce