I think you may be Ok unless the damage to the nylon head has seriously damaged the O ring groove as the head is slightly smaller diameter than the cylinder. The pin(s) are certainly just a single pin. For a new one look at an old HSS drill bit rather than spend a lot on a new one from a spares supplier.
To put rifle back together can be awkward (as above). I either clamp the trigger block in a padded vice and offer up the cylinder or the reverse .. clamp the cylinder and offer up the trigger block. I do not like the 'rest it on the floor and use bodyweight to push down' method.
Cheers, Phil