One of the things I really like about AMP bikes is that all of the pivot parts and damper internals are common items you should be able to find at a good hardware store. So you ought to be able to easily and inexpensively put together a rebuild kit for your fork without any involvement from AMP. If it turns out you can't find the parts needed for your fork where you are shoot me a PM and I can put together and forward to you a fork rebuild kit as I've done for some other Retrobikers here.
As for installing a more modern fork you shouldn't have a problem there either as AMPs have always had 1 1/8" headtubes. I would just suggest going with a 63-80mm travel fork to preserve the geometry of the frame.
Be sure to post some pics in the Reader's Bikes forum when you've got it finished!
"...if 4130 were discovered tomorrow, it would be hailed as the greatest building material in the history of mankind." - Keith Bontrager