I would try to get the valve to move back up with the spring plate first. Do you have access to a slide hammer? Try to get it moving and get some penetrating oil down the stem. It moved down, it could move back.
I'm going on the idea that it was running on the other barrels all the time since you got it and the valve had been stuck a long time and could have originally stuck from lack of running for a long period. If you can get it moving again and put a little top oil in your fuel, you might dodge the bullet. I've had a similar situation with a car engine once and once it was moving again, never had another episode. I'd try that first. That's if you're not out there pulling the head right now.
On edit, it looks like the lift on that valve is now farther than normal. I would not crank that engine, except with the plugs out, and then, very carefully by hand until the valve is clear of the piston. If the valve is not clear, the piston might start it moving back up. Go easy. I'd try pulling it up with a slide hammer. I have made one for a job before. It's not too hard to do if you have a lot of junk laying around.