To convert the bobby car to electric , I did remove the original plastic wheels and replaced them with 6.5" BLDC hub motors which can be found on hoverboards.

First, I used a saw to remove some plastic from there area where the original rear wheels were and made 2 aluminium plates, one with 2 grooves for the motors and a lot of screw holes.

The grooves and M8 screw holes are needed for the hub motors:

The aluminium plates are screw onto a wooden block, that is also mounted on the plastic frame of the original Bobby-Car.

The bottom of the whole Bobby-Car now looks like this:

The cutout in the middle is needed for the electronics, which will be added later.