I think the first place to go is here
http://www.pspec.com/default.asp If Dave Shadel cannot help, then you may have to start begging any other old pylon racers to see if they have part available. Nelsons are no longer made, let alone the older F1 and FAI engines. All good things must come to an end. The Nelson engines will be badly missed when they are all gone. They are something very special.