You may have trouble getting specific information for the Champion as they are quite complicated and also rare, so few will have tackled repairing them. Why not consider its relative, the Record Jumbo, as a possible model, as it too has the oval piston seal, which would have been made out of the same material? Jimmy Dee has provided an excellent stripdown guide for the Jumbo and explains how he dealt with the problem of a crumbling seal https://www.facebook.com/notes/jimmi...34282330002971. He had one machined specially, so you could contact him (he is on this forum, as JimmieDee) for more information. I personally would make my own seal from PTFE sheet and a Stanley knife. It might not be as efficient as an original, but at least you would be able to get the gun working.