Trying to revive Subaru EA190V exhaust valve adjustment screw missing

I recently picked up an orphan Subaru EA190V pressure washer which I'm trying to save from the landfill. It had obviously been poorly maintained and had the typical old engine problems- bad gas in the tank, fuel lines rotten, rust in the carb from water sitting, dead battery.
There were some signs someone had tried to work on it at some point- carb o-ring was reinstalled improperly and pinched outside the bowl...
I can get it to turn over but noticed that it was probably the loudest 4 stroke engine I've ever heard. It then stalls after several seconds.
I took apart the exhaust to look for anything strange then went to check valve clearance and noticed that the exhaust valve adjustment screw and nut are missing. I don't see any damage to the engine so I'm hoping someone removed it.
It looks fairly inexpensive to buy the part but I'm wondering if that would cause the strange exhaust sound and inability for the engine to idle- I've had valves out of adjustment, but I've never seen one with the adjust screw totally gone.
 
I'm a mere small engine tinkerer, not a professional by any standard so any advice is appreciated.