Michigan 75 B shifting issues

Hello, I have a Michigan 75 B? (The plate says 75GM) with 4-way bucket It came with the ranch I bought but I'm not much of a heavy equipment mechanic. Turns out that I use it quite often for a variety of tasks. Lately when I start it and shift into gear it moves a short distance then stops, but by manually moving what I believe to be the shift valve with my hand seems to fix it for the day. Today it did the same thing, but when I crawled in back to move the the shift valve it doesn't seem to affect it any longer. No background, wrenching on heavy equipment, but it appears that the little (shift?)valve on the transmission has failed. Trying to find the correct parts and service books, without success so far. Hoping to find someone with a bit of experience working on these old loaders, where to order parts, etc. Had a great old guy with a service truck but he's no longer with us. Any advice or remarks are welcome. Thanks