From what I see of the stock, it's split where lots do. Could be more time economical to find a replacement as a better base to begin from.
As for the piston, it simply looks like the seals have disintegrated with age and need replacing. Other than being dirty, the actual piston looks fine. Take all the seal components apart and then piece them together off the piston, this will give you a better idea of what's what.
Perhaps this video may help:
https://www.youtube.com/watch?v=3yeZ...=CharlieTango1