I would run a compression test on all cylinders to start with. When you pull the plugs to do the compression tests pay attention to which plugs came from which cylinders. The plugs can tell a lot about the health of an engine. If any are black or wet, these are the cylinders that are probably not working properly. You can start to trouble shoot from there.
To be honest in my 40 years of working on cars/engines, with the exception of engines that sat for a long time in the weather I dont ever recall a completly stuck valve. I have seen sticky valves that oil additives would generally cure but I dont ever recall a completely stuck open valve.
Run the compression test and let us know the results.
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