Service manual crossover

After using search function here looking for a service manual for early 112 3u, it is thought they never made one. However they said to use one from something similar to 112 3u. What model / series would it be?
Wanting to rebuild the power control box, I think every seal in it is gone. Trying to decide if I can do this myself or take to a shop. While in will fix whatever it needs.
Thanks, Andy.