Don't take my view as gospel but I have done the Lazarus routine on motorcycles for years.
Some were "resting" for a long time too and stuck.
Pull the plugs and make a mixture of 50/50 acetone and ATF. Acetone is nasty stuff so be careful. Pour down each plug hole.
Some would say drain engine. Put plug back in and then fill with kerosene or diesel. Let it sit too.
After a week say , try putting it in 5th gear and rocking. If still stuck, wait a week. Try again.
Typically the pistons are stuck in calipers and the pads may have been rusted to the discs. I would take a rubber mallet and bang them to "shock" loose. If they have rusted to the disc, then rocking may break loose. You WILL need to go through the whole system and rebuild.
A good washing is the best way to start. Look underneath while you have it on jack stands (rebuilding brake system!). Examine closely for rust.
Polish paint and stand back and look at it.A guy I know who does vintage Porsches goes over the paint with 2000 grit sand paper.
Too many people fly into sleepers like this and strip them, farm out bits and then burn out.
If I could break engine free, I'd then drain all of the kerosene & 50/50 solution and then put something like Marvel Mystery down the cylinders and spin with a battery.
Go through carbs and rebuild .Install new oil and filter and then do a remote gas tank feed and try to start.
You'd be surprised.
Years ago I dug out an old Honda 750 motorcycle . It was an early one. Sat for years. It was stuck. I did all of the above and it fired off. Non of the rings were stuck. If it had of started smoking, I would have shot Kroil oil into carbs. It has been know to free rings up when running.
Later Americn Honda bought this bike from me because it was SOOO original. Still in there museum.
Oh yeah. Be patient.