As a starting point I'd fit the later longer stroked and lighter weight synthetic sealed piston, a piston liner coupled with Delrin guides and top hat and see what the firing cycle is like.
Piston weights and strokes :-
http://www.airgunbbs.com/showthread....99#post7168099
If you have a 4mm transfer port on the gun you may want to consider sleeving the transfer port down to 2.8mm if it feels a bit slammy with the above tune.
All the best Mick