Chrysler is slowing my Dart build

Except the torn servo piston was from an A413 (in Voyager), which is basically the A904 in a transaxle (descendant of A404). I have read of similar "torn servo seal" problems in A904/727 transmissions and an after-market piston (Sonnax?) that fixes it. However, I can't find it anywhere. Does anybody know of such, and have the dimensions so I can check if will fit my A413 (upcoming job)? Some Neon "tuners" replace their torn servo seals regularly.