After some stressful daydreaming about what the F am I going to do about the seized engine, I did some research (mostly on the Moyer Marine forum) and found that it is possible to un-seize an engine (previously I thought they were junk when they seized). What you do is pour Automatic Transmission Fluid in the cylinders, let it soak for as long as you can stand to do nothing/something else and then try to crank the engine by hand.
Cranking my engine by hand proved impossible, but since the starter is already off, I was able to use a screwdriver as a lever on the flywheel and force that bastard to turn. It was not easy, but I got 'er done.
And now it's turning smooth as can be. I was also able to see how the valves work and which ones were not working. In the pics, the larger valves are the exhaust valves (allowing greenhouse gases to leave) and the smaller are the intake valve (allowing air/fuel mix from the carburetor). On cylinder one both valves were working. On #2 I had just the intake (but eventually got the exhaust to work too). For #3, I got nothing. And #4 has just intake working.
I think the next step is to take the valve cover off and have a look at the valves. I'm pretty sure the valve cover has not been off, so whatever junk is down at the bottom of the valves is pretty filthy after nearly 40 years.
I have to say, I feel puh-retty manly getting in there and making engine stuff work again.
Here are some pics of the engine going through all of its cycles. Note the position of the pistons and the valves. Engines are pretty complex machines and I wish I had appreciated them sooner. I also include two pics of the cabin so you can see what a got-damn disaster it is down there.