Many of the issues you describe i have run into myself, having both the TM A1 and A2.
to fix the loud sound of the aug take a sock (preferably clean) and cut it to the length of a small battery so it is a tube, and then put the battery in it, then back into the gun. problem solved!
forward handle pin can be replaced and is quite easy to get as a special order part from airsoftparts.ca
Hopup does suck, so i just finished (like 10 minutes ago) installing the g&p metal one. seems good, but make sure you have silicone oil or you will never get the rubber in the hopup.
you may need to go to the mosfet, but as an stand-in until then take the rear plate off the aug and where the mechbox touches the plastic plate take a small piece of cardboard (like from package of gum) and tap it there, this pushes the mechbox forward, and tends to fix this issue, which has occured in my a1.
gearbox removal is best illistrated here
http://www.mechbox.com/site/aug-upgr...bly-video.html
I love my AUGs which is why i own 2.