Hi,
This might not be as simple as just a new set of pistons.
It is going to depend on cylinder wall condition, and current size of said cylinder bores.
I dont know how many engines you have done, or your experience level at this kind of thing. But the cylinder walls wear larger over the course of miles. Your walls are also scored, so depending upon bore size and damage, you might be able to get by with a hone and a new set of pistons.
If they are in tough shape, might have to have cylinders bored over-sized and then buy new plus sized pistons to go with them.
Then, you need to figure out how or why we welded a piston tight.
More pictures, we love pictures!
Lee