I don't think you will be able to find a pipe for your model year. Performance wise, you would be better off selling your bike and use your extra 800$ to get a bike a bit more recent. I'm saying that because in the 80s, MX technology improved so rapidly every year that often the last model year yould not even be raceable. You will notice that in the 90s, the changes were more subtle and sometimes they were even steps backwards. Good luck!
I agree with that. You could find a decent bike in the early 90's up to 95 for between $1000-$1500 and you would benefit from the more modern layout of the machine and better suspension. Check out cycletrader.com and do a search for bikes in your price range. Here is an example.
If you really want to do some improvements to that bike instead of buying another, the single best mod you can do is to modify the suspension to best suit your weight, skill level, and terrain that you ride in. Proper suspension action will greatly enhance your control and comfort, and will improve your speed more than any engine mods you can do.