A long time ago in a galaxy far, far away . . . well, a month or so ago somewhere near Stockport I spent some more time with the honorable Neil-san. The end result was mounted motors.
To beef up the hull where the final drive is bolted to the sideplate a 10mm ply doubler was added.

Doubler

As you can see from the underside view, the doubler has been keyed into the hull bottom to add a bit of strength.

Underside

There will now be an aluminium mount added inboard of the sideplate with a bearing for the rear axle and a locking mechanism to hold the motor in place.