stuck motor

[nhusa]

15 years without being started on an unknown rebuild requires more than a bit of penetrating oil and a big wrench or hammer.

Remove the head, valve cover, and pan.
Then inspect things.
After that you can begin using penetrating oil - on the cylinder walls and valve guides.
I would also find a way to get oil into the bearings - maybe spin the oil pump with oil in the pan.

You have no clue about what the rebuilder did or did not do.