Since this is a model of a prototype vehicle built by the Top Gear team it's going to need a big engine. This one is a whopper. A Graupner motor gearbox unit that should be capable of shooting this vessel forward at breakneck speed, or at least give it the ability to trouble the skin of any passing rice pudding.
Fitting was surprisingly easy. Drill a hole in the back of the tub that forms the interior and poke the shaft through. A couple of bass wood lumps carved a bit are glued under the lugs sticking out of the sides of the gearbox. More glue (5 Star adhesive epoxy) seals the shaft/plastic exit.
I wasn't sure what size propeller to use. The unit comes with a 30mm one which I worried might bash the bottom of the model so a 20mm version was put on order. However I think the larger version will be OK as it just clears the plastic bits. The engine cover (a part I wish was fitted to early Bay Window vans, especially the 1971 model) isn't fitted until I am confident that the prop is OK.
You might have guessed that this is all a bit unplanned. I make something and see if it fits. Perhaps some working drawings and proper planning would be better but this is a fun project. Besides, I'm rubbish at drawings.