Several years ago I had one like yours. When I drained the oil antifreeze ran out first. It was stuck fast. I removed the head to find one of the cylinders had coolant standing in it about 2" deep on top of the piston. Those rings were rusted to the cylinder. It had a cracked head. I got another block and head and built-up a motor with parts from about 5 motors.
The 216 was stuck in the 50 Chevy I have now when I got it. I drained the oil and flushed out the pan/sump with diesel. I did not find any water. I removed the spark plugs and applied a generic light penetrating oil about the weight of 3in1 or sewing machine oil. After a few days I did it again. I did this almost daily until I ran out of that stuff then I went to Marvels. I waited about 3 weeks before I tried to turn it over. I put on a new battery and it turned over fine. I am running this motor now.
Some of the old tractor websites have remedies for freeing stuck engines. Motorcycle guys run into this too. Good luck!

Professional Novice