Welcome to IFG Arshad!
Do keep in mind, it's not all about FPS. FPS will increase the appropriate amount when other aspects are properly taken care of. Aim should be to make it shoot nice & tight, the right balance of FPS & accuracy. Here's what you can do.
- You need a spring compressor to safely open it. Make one.
- Take it all apart & clean all the individual parts. Use kerosene or such to remove the greasy muck.
- Polish piston & chamber to a non-mirror finish.
- Try get a synthetic washer of the right size. Not sure what M23 is like.
- Cut a few coils of mainspring if cocking is hard & grind mainspring ends to flat finish. Assuming stock spring is decent. Preload of ~1" is enough.
- Machine a new slip-fit spring guide or properly sleeve the current one. Consider a snug-fit top hat too.
- If you wanna go all the way, consider thrust bearing for your piston.
- Install nitrile rubber O-ring breech seal. Let the trigger be, it's not very safe to fiddle with it if new.
- Get 60% moly paste & lube everything inside properly. Assemble it all back properly & let it settle in a few hundred shots.
You'll know it's a good job if the vibrations & noises come down substantially which itself should result in better accuracy & power.
You can find ideas & some pics about most of the things mentioned above including the spring compressor over here:
viewtopic.php?f=2&t=21308 . Search IFG for even more tips.
All the best.