If the part isn't that rare, I would junk it and find a better one. As stated previously, rust pitting on the gears will make the transmission louder than normal. To bad you are on the other side of North America, I have a couple nice SM420's sitting around waiting for someone in need. If you want to free it up just for fun, filling it up with old drain oil and letting it sit for a few weeks or months will usually do the job...no heating or beating necessary.
