Well, the fact that you can keep it running by invoking the "mow in reverse" over rule switch tells me that the issue is likely the reverse sensing switch has failed in the applied position, or a wire serving that sensor/switch has shorted to ground, which mimics the condition of shifting to reverse gear. Or in other words, it thinks you have selected REVERSE travel.... all the time.
If you can locate the reverse sensing switch and wiring you can probably/maybe solve the problem without an actual diagram.

Ok, thanks for the info, I will see what I can do.

My "guess", not having a schematic, is a bad, misadjusted or disconnected seat switch.
EDIT- mownie may have a schematic and he IS knowledgeable about these, so pay attention to him first-

This post was edited by bill_kapaun on Mon, Mar 31, 14 at 14:01