Welcome from Lake Superior,
There are methods for unfreezing motors while they are still in the bike. Pull the plugs and squirt in oil or mystery oil to lube the rings. Let stand for several days. With the plugs out try turning the motor a little bit in both directions. Hopefully you will have movement, so just keep working it in the correct direction until you get a full revolution.
My 550 sat under a tarp outdoors for twenty years. I brought it back to life without having to pull the head.
-MD