Judging by the look of the geartrain /drive mechanism, I suspect that a model engineer (or, someone with those skills) has been involved at some point.
I think I would start by applying some voltage (say, 6v) to a few test points around the various circuits. Looks like the speed controller may be of the 'resistive mat' type (a bit like the old "Bob's Board" ). From the photos, this seems to be just below the rower's seat on the foredeck.
You could also trace the wiring back from the on/off switch and this should lead you to a fuse or, battery connector of some type.If you have a multimeter, set it to read "Ohms" and look for complete circuits.
Probably best to understand all this before delving into the radio control side. BTW: I doubt this is a commercial product….
Have fun !
Dave
Just spotted the yellow item below the on/off switch. Looks like it could be a battery pack. Maybe dry cells or, re-chargeables. This should give you the operating voltage anyway and maybe OK for testing.
Edited By Dave Cooper 6 on 14/01/2022 11:00:40