Hi Larry,
Sounds like you have a real problem there, my 76 CB750F sat in my garage for 12 years without being touched and it spins freely. But when I stripped it I found a nipped up top compression ring on No. 2 cyclinder.
I did buy a 77 CB750F2 that had sat outside in the English weather for 2 years and was locked solid, took the motor out and spent a long time with WD40 and large rubber mallet/pieces of wood to strip it down. Had to do a complete top end overhaul, new rings pistons etc.
If it were mine and I was in no hurry I would pull the plugs and squirt WD40 into the cylinders and periodically try to turn the engine. But not that I am no expert, presumably if the engine used to spin up and it has not been run up there should be no problem with the bottom end or gearbox.
However even if you free it off, you will not know the condition of the pistons/rings and cylinders. If it does free off do a compression test to try to gauge how healthy it is.
There may be better additives out there but I do not know of any.
Good luck