In the '50s, we ran the K&B 29 on pressure. As I remember, drill a #80 hole through the threaded tank mount. I found a "universal" needle valve with the proper threads to fit the backplate threads.
Used for stunt, with all tank vents closed. It's curious that it works, because crankcase pressure goes negative then positive each cycle. I guess there is a net positive pressure, which keeps the tank at a slightly positive pressure.