I've delt to quite a few seized engines. First determine if it is seized from sitting around, mechaniacl damage or mechanical damage then sitting around. Has it got water in the oil ect. ect. Filling it up with straight Diesel will test the crankseals plus displace water which can be dropped from sump or my lazy tube down the dipstick hole,(copper or Bundyworks), if petrol spark plugs out and equal parts mix of Kerosene/ATF/Hydrualic oil. Yes I know ATF & Hydrualic oil are essentially the same but most Hydrualic oils are Hydrophobic & ATF has gum solvents & detergents & the Kero is just a very light Diesel oil with great penertration. Anyway the three work well together & are not overly volitile.
Squirt some down each plug hole and let soak. Pressurising is good if the valves are seated but you need to be able to turn over to do each cylinder.
After a good long soak try and bar over. The flywheel ring gear is a good place. Try one way then the other. If no movement soak longer. When you get ANY movement bar it back & forth but not too aggressivley.
I've taken 2 -3 months to free up a badly seized engine, particulary bad if they have been shut off hot with heat seizure & parked up for a decade or two.
You might end up having to strip it down but if you can get things moving life will definately be easier for the effort.
I'm on month 4 of a Bedford 214 engine that has blown crankseals, blown head gasket & I suspect broken rings & at least three cracked pistons. I have a new engine but want to free up this one in the hope something will be salvageable, it has only just started to move and I just hate to be beat.