I used to repair them for our stuff over a decade ago, near forgotten them ... until about a month or so ago when I cleaned out the shed, and threw 4 of them out.... I will never need them again..... so I don't have one to go over with you.
I forget too much now.. but. You have speed control, so the pwm chip ( second in from bottom right corner.......... relays to the top) is fine, and that is the only component not replaceable.. it is a pmc chip.... all the rest are off the shelf chips.
The 339 beside the pmc chip is fine, it pwm's the fets. The totems are fine, they drive the fets... so your problems are that the transistor that turns on the reverse relay is probably toast..... up near the relays..... forget how they drive them now, but I kinda remember they had micro switches in some models for F/R, and the pot controlled fwr/rev on others.... your sounds like the latter.... check all things off board that can cause reverse to kick in too... micro switches if it uses them for f/r.
So find the tranny that controls the rev relay ( pop the top of the relays if you have to to check for fused contacts too), and find out if it is short, or the driver for it is causing the trouble.... trace back to find the comparator it uses to control this function, it may be shot, or the voltage set level may be wrong for whatever reason, so all pot settings make it think reverse... that kind of thing is what we need.
The fact you have speed control is good, that means the relay away from the other two is ok, as that turns on the power... so there is not much to check.... but I recall it is convoluted sequence there for the pot loss and direction.... a CRO is almost a must for this stuff.... it can be really simple... or can be days to find a simple problem, but most of your board is fine, so thats a ripper start..
There are no flywheel diodes, as it uses the fets to achieve this too..... this is why you have 8 where four would have been enough... so if your circuit tracing, this may explain a bit... they turn on when the drivers turn off etc etc etc cheaper than diodes, but comes with frailties too.
best of luck, and sorry I no longer have a board to remember from.....would only be a few months I'm sure.... dammit.