I did fix up the coupler. Its a prototypical tongue and socket with a pin.... I made mine from K&S telescoping brass tube for ease of manufacture.....
I re-engineered a taper pin and a sleeve around the tongue to stop it when its in the socket far enough for the holes to align.
In the real thing there is a double ended socket with tongues on trailer and motor with a pin on each.... Would have been easy enough to make but hey the loose sockets would soon get lost....
The D shackle is out of a box of old marine fittings, it looks the part here and is about the right size.... I think....
The black wire is the jumper for lights to the trailer.....