3 spped manual trans when unable to get it into 1st and rev - check the screw on the side, under the top shift lever. It's a slotted screw head. after removal check the length of the shoulder past the end of the threads - should be about 1/4" if it's less - it's sheared off. If the pin shoulder is sheared, it allows the vertical shaft to move up, instead of the internal lever, so it can't engage the 1st/rev rake shaft.
Use a magnet to find the sheared end (in the lube).
Can't help in finding a new screw - I had one in another old tranny.