If it is like a Briggs and Stratton reduction gearbox, the gearbox attaches to the crankcase side-cover, so you would have to remove the side cover (including one of the crankshaft main bearings) and fit the whole unit to the other engine. As far as I can tell that is a side valve engine, which as far as Honda is concerned, pretty much went out of fashion a long time ago. In other words I don't know the answer, but given Japanese manufacturers' tendency to keep redesigning their products, plus the fact that you would probably be buying an OHV or OHC engine that has a rather different crankcase from yours, I think it is very unlikely it would fit onto a modern engine. If you can find the engine's model number (probably on the red cowling) you could ask a dealer, or the part number may be shown in one of the parts lists available on line.
