Piston Stuck
Rebuilding means boring it and fitting new pistons. So take it to a machine shop and they will separate the pistons and rods, usually with catastrophic results to the pistons.
If you desire to reuse the pistons, and they measure/test as reusable, AND the bores are ROUND and not tapered,and within usable limits,
Then, this is called reconditioning.
In your case the piston-pins are captured in the pistons during the assembly process,and the pistons are usually broken or otherwise damaged beyond reusing, during the disassembly process.. The pins have a small clearance in the rod-bushings. Yours appear to be varnished up. So you will need a chemical to dissolve that varnish.