my computer just spits out the scratchy old install disk
Well, that might keep you from using my normal response - Repair Install, or maybe not.
You will have to fill in some blanks, as I do not know Mac software. On the PC side of the street, there are several DVD burning utilities that make copies of DVD's. As odd as it sounds, I've been able to make good copies from some crappy DVD's and CD's. Some of this might have to do with your particular drive. Different drives read different discs better than others. My workstation has a Plextor and an LG. I have found that with certain "bad" DVD's one will do a much better job, than the other. It seems to be totally disc dependent. Normally, I put a bad disc into the LG, just because it more often can read a bad disc. However, there are some that the Plextor handles nicely, while the LG chokes. I use one of several DVD copy utilities, but usually grab Nero first. Do not know the Mac equivilent, but there have to be many. You may need to try your install disc in several burners, before you find one that works with it. This is what "friends are for," right? Since you are doing a byte for byte copy, you should be able to do this on a PC. It'll be the burner that will be the issue.
If you get to the point that none work for you, and have not found another solution, I'd be glad to give it a go on my system. No promises, as the disc could just flat be shot. Still, I've only found a few that I could not copy.
Last thought: there is a clearing house for older software. These are supposed to be fully legal versions. They do have some copies of older Premiere, though I do not recall if they had Pr 6.5 Mac in their list. I can't find the URL right now, but it appeared in a post by John T. Smith in one of the older version fora. Title of the OP was something like PrPro and AfterEffects... If I find it, I'll post the link to that site. I cannot vouch for them, as I have never used them.
I've also not heard of any of the later versions of Pr breaking older versions. That is a bummer.
OK, here's the LINK to the "old software" site. Like I said, I have never dealt with them, but if you had to purchase at a fair price, Pr 6.5 Mac, this might be a good source.
I do know that Adobe will sell replacement discs for registered copies of their programs. How far back they can go is anybody's guess. I would assume that they keep a master for most of it, and would just copy that to send out, but do not know. I would contact Adobe Sales, but have your S/N and registration data handy to save time.