Diy petrol board questions

hello. I’m in the process of building my own petrol powered board. Im planning on using a minarelli scooter engine, cut every unecessary parts and leave just the cylinder and crankshaft housing, making it direct-driven.
Here comes a couple of issues. I can’t seem to find any info what the various boards use for an exhaust check valve, and after some google searches i didn’t come up with anything good. What would be your recomandations?