On the MK1 Airsporter there's a nut inside the piston holding the piston rod inside the body, once you've removed the nut you can pull the rod and seal out of the body.
The nut is a barrel nut with two cutouts in it so you'll need to make a tool to remove it --- I cut a bit of thick wall tube to fit the nut.
I think Knibbs sell a leather replacement seal, if not they are the same 1.1" OD as a BSA standard seal.
All the best Mick