Two wheels, one free, the other one connected to the engine.

Clutch can either be integrated on the engine (if using chainsaw engine), or a simple belt tensioning system.

Things are getting a tad clearer.

Driveshaft and wheels

The wheels should be decided by now, and will use a 20mm shaft. This helps a lot designing the driveshaft itself. Now, we can't drive both rear wheels, as it'll be a problem whenever we'll take a turn. Instead, only one wheel will be coupled to the driveshaft (through a custom coupler), the other one will just sit on its bearings and spin freely.

The entire shaft should be held through two bearings on pillow blocks, pretty easy to bolt onto the frame.

Engine

We'll probably end up using a Husqvarna 165RX engine, 65cm³, about 3HP. Pretty light for that power. We'll have to adapt something on the output shaft.

The engine's backplate looks like this:

I'll probably find some tubing that adapts on this coupling. It'll be directly welded on a square pipe. The output shaft (press-fit onto the clutch bell) will go through it, and will be held by a couple of bearings with pillow blocks.

Second choice would be a Stihl 25cc engine: less poweful, but with the right gear ratio, we might have enough torque to push it up to 10km/h...

Transmission

The power from the engine shaft is transmitted to the wheels through a sprocket. Probably something like this.

The sprocket's size on the drive shaft has yet to be determined, but apparently a 5:1 ratio seems sound (according to this source). A custom coupler will have to be turned on the mill to match the 20mm driveshaft.

We'll try to use a T8F chain, widespread standard (and it's metric!). That or a #35 chain, seens to be pretty cheap too. @Frédéric Druppel also has a #40 sprocket, we'll have to choose between those three possibilities.

Braking

Another "urgent" (ahah) matter. eBay provides lots of cheap disc brakes and calipers for bikes, for around 10€... Another small coupler will have to be turned to mount it onto the driveshaft.

As for now, the priority is set on the rear shaft itself: the wheels, the axis, the bearings and the braking system.