I know a guy who made a steam engine and it worked with air also.I think the vibration would be really bad for a 1 cylinder.For the valve you couldnt find would it be possible to use something like a pipe with a solid shaft in it, then drill a hole through both, pipe would be solid on 1 end and have something to hook up to a servo to move the shaft in or out. It would take alittle time to experiment as to what size would work or be needed.If it was small enough it should reduce pressure. If you can do machine work you could also cap open end and o-ring it to stop leakage. also drill and tap hole on pipe to have in and out connections. You dont have to stop doing it , just get boat running now with whatever you decide, but keep working on the air motor for a later project. You may find help if you can find any steam engine plans. Good Luck and your boat looks great.