Now I will most likely be shouted down on my views on an old model such as you have, but: why change it?
OK, leaky breach to be mended if it needs it ... really need to chrono and eliminate all other possibilities if the power is way down. No harm in replacing service parts as well if needed.
But then that is only my take on it ... and I can understand your desire to concentrate on the rifle as an instrument and to increase its efficiency rather than as a piece of airgun history as it is.
No idea about Tbt kits. Also no idea on real effects of updating the piston seal other than I believe just doing the seal with no other changes is not really recommended.
Cheers, Phil