Repairing your old pipe is not such a major issue if you can locate a good motorcycle exhaust specialist and I would urge you to do this as these pipes are irreplacable. Sure you can get a new set of HM300s but unstamped early pipes are unstamped early pipes. Getting to the rust inside the pipe is no real drama, the drama is sealing the rust holes and outlets so the rust cleaner can do it's work. I find black sikaflex is the ducks nuts for this job. You can get this or similar product from a marine specialty house. There are 3 ways I use to chemically clean rust off metal from the inside, and this will also work for tanks, side and centre stands, bolts and even your tools. First method is to used a mild solution of acid, however unless you are expert I would not recommend this as you have to know when to stop. AM has this problem when he sees a carton of Fosters. Second is to use a product called "rustoff" which is available from specialty automotive and paint houses. This works a treat and will clean the stains off grandmas undies. Just the trick for that tank as well. It works through chemical reaction (I'm right up there with my Chemistry) with the rust and leaves the metal with a dull sheen. Third method, and very effective, is to use molasses. This is as good as anything around to clean rust off steel and iron. It will need to be left in contact with the metal from 1 to 3 weeks. Mix the molasses 1/2 and 1/2 with water. May take a little effort to find though.
Finally, and after you have done a thorough rinsing in water to remove any trace of the rustoff or molasses, YOU MUST use a non acidic rust converter to neutralise further chemical reactions and stop any rust formation.
You're now ready to take your pipe in for repair or rechroming. If this seems all too hard, when you toss those old unstamped pipes out .... pick me KP
