I wouldn't waste my time rebuilding a SM420. There are so many good used ones around and most of them sell for practically nothing. That is the reason parts are difficult to find. We ran quite a few SM420s back in the '50s and 60's. The only one I remember failing was a PTO leaked so much oil that it ran dry. We went out and bought a used one like everyone else did back then. I'm not sure what you mean by a little lurching down hill and whatever it is probably wouldn't be fixed by a complete transmission rebuild. What kind of experience does the guy that suggested going through the transmission have?