OK, you got a badly burned exhaust valve. You can get ANY old exhaust valve you got laying around (or a new one) and lap it in. I found the easierst way to quickly cut a valve is to insert valve in head and chuck an electric drill to the shank. usae a sharpy to mark valve seat and valve lip, apply lapping compound and spin valve at med speed while making short light contacts with the valve seat, sort of an up and down motion. Reverse the drill every few seconds. You just want to cut the crud on the seat with the valve angle. Once you see the sharpy uniforrmly scrubbed, you can remove all traces of lapping compound and reassemble springs. I use a steel wool pad to get surface crud off seat prior to all this. A new valve will have proper angle already on it. An old valve may have a cupped lip, not good. Use a good core or new valve, they are cheap, and a machine shop will cut a bare valve for about $1-2 a piece.
As for your variable compression readings, you cant measure a hydro tappet motor like this, you need to have them pumped solid from a previous running, warm. You lifters are soft with no fresh oil and you get an incorrect compression reading. Slants are very durable (besides burning valves I guess!) as stated, get it running warm with 4 quarts walmart oil, and add a quart of kerosene or motor flush. Run it for 15 minutes idle or whatever motor flush says WHILE slowly dribbling water into the carb at a high idle, the water will steam clean your combustion chambers, and then drain. Itll be nasty stuff. refill and change filter and you should be good to go for the next 500 miles, then repeat.