HBAR front ends are pretty hard to find but do pop up on shops like Dentrinity or Redwolf. Finding a used kit of a forum is also a good bet.
If you want a good cheap tightbore,Madbull and ASG Ultimate(OEM by Lonex) are good bets. Found pretty much everywhere. ZCI also make steel barrels,which are known to have fantastic internal finishing and machining quality,which is compensated by straightness not being as good. The internal finish has been found to be better than Laylax in some cases. Can be found on ak2m4.co.uk,
JG are not known for precision casting,but their hop up chambers are adequate. If you want a new chamber G&P make a good metal one.
Good buckings- Lonex 70 degree buckings and G&G greens are good and available. Systema are decent but these two are cheaper and better.
ZCI 22TPA high torque motor is fantastic for trigger response. It has better build quality than Chaoli or JG 22TPA motors. The magnets are very close to the armature,closer than you would normally find in a high torque. You can find it on ak2m4.co.uk,owner is sound and on this forum I believe. Prices are good and he stocks a lot of under rated parts like ZCI which are fantastic bang for buck.
You can leave the rest of the gearbox alone. JG are know to make fantastic stock internals.Pistons are strong,piston heads seal amazingly,spring guides are great and shells are also beefy. Gears are also incredibly strong,some of the strongest you will find stock.
If you further wish to improve trigger response,you can grab some SHS 13:1 gears,paired with a 22TPA motor these will give great response.
Any gearbox work I would further recommend is correcting piston angle of engagement using sorbothane pads,increases the life of the piston drastically. Use some PTFE tape to seal up the cylinder head too.
I'd re grease the gun since JG's grease is pretty awful in my experiences
Of course,with a new motor you must correct the shimming. Make sure bevel to pinion meshing is spot on. Easy enough to do on V3 gearboxes.