That one is an oldie. Can you describe the problem it is having?

If it needs more than replacement of the pulleys, inner plate or a pivot spring probably not. Per my older Shimano parts book the main body and parallelogram are shown as a single assembly on most. With current labor costs most derailleurs are considered disposable items.

The best DIY derailleur repair instructions I have seen are in the old Glenn's bike manuals. Two editions exist, "Glenn's Complete Bicycle Manual" of 1973 and "Glenn's New Complete Bicycle Manual" from about 1985. Both are available used on Amazon for reasonable prices.

Otherwise check your best LBS to see if someone can help you or keep an eye out on eBay for the correct replacement.