There may be slack in the cable causing your problem. Have someone give you a hand and move the selector from neutral to forward making sure the that the level on the trans mission goes to the forward position. Then move the selector to neutral and check the position. Do the same for reverse. you make need to adjust or replace the cable to get a solution.

They haven't made that trans for close to 40 years and parts are no longer available. If you want the manual linkage you need to find a used trans that has it on it. I would try cleaning up and lubricating the moving parts where the shift motor operates the trans shift lever. Also you will notice the shift lever on the trans is mounted to a cam which operates the reverse lock valve. That cam need to be kept greased. Also remove the little cover and check that the 3 wires are connected well. I actually have 3 of those electric shift motors I got off trannys I bought. Do you have a manual on that tranny?? I have one in PDF if you need it. Shows everything about it. Just give me an email address and I'll send it to ya. Where are you located?