Questions about my need to me engine.

[Airshac]

Just getting to know this engine. Coming over from VW buses and aircraft maintenance. The engine seems very easy to work on.
I did a compression check and had 3 cylinders at 70 and one at 65. How is that? I don’t know anything about the history of the engine.

Secondly, I think the thermostat is stuck. I guess by default, if it malfunctions it will stay in the open position. I ran the engine for twenty minutes and water was coming out the exhaust but the water temp gauge never moved. I was concerned to run it longer because I don’t know if there is a way a bad thermostat could cause water to by pass circulating through the engine? Where is a good source for a new thermostat?
Thanks for the help!