Hahn tiller, need help figuring this one out

I have a 1973 Hahn Heavy Duty Rotary Tiller, model #BPDT-124, series 8675, serial no. 1129714, (power reverse model). I used it in early May to rototill my garden. Ran like a champ and no problems. Today I wanted to use it to cultivate between the rows in the garden. I started the engine and moved the clutch lever to the forward position the tines didn't rotate. So I pulled back on the clutch lever to put the tiller in reverse and the tines rotated backwards, (you have to hold the clutch lever back to reverse the tiller. When you let go of the clutch lever the lever moves to the neutral position). After doing a series of forwards and reverses, I pushed the tiller back into the shop. This tiller uses two different belts. One for forward, (tilling), and one for reverse. I thought maybe the forward drive belt might have stretched or something so I put a new belt on it. I started it up in the shop and the tines did move around very slowing at full throttle and if I put in the soil the tines probably wouldn't spin. I know this is an old tiller and probably not very many around any more but this is a beefy unit with a cast iron gear box that I'd like to keep running. I thought about this all afternoon on why it will go in reverse and not forward. Anyone have any ideas or suggestions? I'm at a loss on what could be causing this problem and what happened since I used it last to cause this problem.