To find a new motor, I sacrificed a mini RC car :

And of course, the motor didn't fit in the other toy I had started to work with...

And this mini RC car is too big to fit "in" the mushroom,so I just kept the rear of the car with the 2 wheels. Then I dug in the mushroom bottom with a Dremel and used a glue pistol to keep everything in place. On the picture you can see I had to cut the corners of the PCB so it will fit in the mushroom!

There is only 2 wheels, there is no room to easily add a third wheel to stabilize the mushroom, so I used some tape and plastic (yeah I know...) to make some kind of "skate" so it would be more horizontal. This is really not satisfactory, but I wanted to try to have something working before the end of the coin cell challenge.

Now the question is, does it work? Well, it works with a 3,7V Nimh battery, but not with a 3V coin cell :-(
I have a couple of coin cells at home but they don't seem to have enough juice to move the mushroom, so I'll try to get new ones tomorrow to see if it can work.

I'm a bit skeptical though, the RC car weighted 19g and the hacked mushroom weight 49g and there is much more friction because it has only 2 wheels.

Another problem with having only 2 wheels, is that it doesn't really go straight, so I tried to make some kind of ramp, or rail in balsa, with borders to prevent it form escaping. It works so-so, but I started to add a brick pattern so at least it will look nicer ;-)