My HW77 has a modified stock with hamster, adjustable comb and butt pad, and the pistol grip has been reproflied to suit my clunking big hands!
internally it runs a 25mm piston with delrin bearings in a 26mm compression tube with matching piston seal, V-mach spring/guide etc, the barrel is shortened to 15" (to keep the length within reason) and a still air/underlever latch/muzzle weight I turned up out of silver steel!
its doing a 15mm five shot group at 55yds down our range and 36mm at 100yds when i tried it for the hell of it, thats using Webley Mossies.
in reality however you dont need to go to the trouble , the difference between my 77 and an out of the box TX200 is not all that much, it's just i like the firing cycle to be quick and I find TX's a little lazy feeling!
oh and I like making things just for the hell of it