The tappet isnt very hard to replace you just have to take apart the gearbox and replcae it. Its the part that moves the nozzle back and forth to let bb's into the chamber. The only hard part about it is that you have to get the timing right, and the psg1 has a different timing than all other aegs.
I sold my PSG1 about 6mo ago. I had installed the full tune up kit and it was a pain. I had to take it down 5 times. I then had to build a 10.8 battery into the stock. The gun looks great but is big and clumsy to cary around the field all day.
Thanks everyone...I have decided against the PSG1 in favor of a M4 varient.