Then tested the motor with a set of small jumper leads to a 12V battery and discovered that the motor is just fine thanks, it runs quite handily at 12V. (Most of these ride-on toys do, they use a 12V motor and run it at 6V to preserve the life of the motor and keep the acceleration low.) A bit of trimming to fit the larger battery in the chassis, a hunk of trailer cable, and I have a small robotics platform that has battery onboard, and a forward/reverse switch on the end of the cable.
It has good ground clearance since I removed the "anti-wheelie" skeg at the back, and the wheels are a bit bigger than the size of a CD. (That's a CD in a paper sleeve leaning against the wheel, for scale.) It had only one driven wheel (and of course it is almost totally bald) but I used a pair of screws to lock both wheels to the axle so now it has twice the traction and goes over the fairly uneven and sandy "lawn" I have here, quite nicely. I may even find some old inner tube and cut two slices of that to put over the hard plastic tyres to provide a bit more traction, but so far the combination of the weight of the battery and the solar panel temporarily fixed onto it seems to be doing the trick.
No steering mechanism yet but that won't take long, and no speed control but all I need for that is an old battery drill that's burnt out its motor or worn out its chuck. And the drill body would serve as a handheld I could mount the reversing switch to as well. (Nice thing about the battery being onboard - no battery for me to lug around...)
I have a 4W solar panel/battery charger combo on it too - so I'll fit a few chunky rails to the chassis and mount the solar panel permanently above it, then find my new robot a job to do. (Not visible in these pics, I removed it again so I could take pics of the guts of it - more pics later when I mount it properly to those chunky rails.) At the moment I'm all in favour of adding a small lawn cutting blade underneath, and making a controller that just moves it around to follow the sun, and cuts grass as long as there's enough juice available. Maybe later I'll find one of those WiFi enabled kits (or hack one together) and then I can use it also to patrol the yard with a wireless camera, mic, I/R sensors and perhaps a speaker and a grabber as well as the mower underneath.
So far the project's cost me $100 for the solar charger, (which was actually bought for another project that never eventuated,) and a few bucks for leftover trailer cable from another job. The battery came out of a dead UPS, and the reversing switch was the "gearshift" of the original ride-on toy, and three or four self-tappers and a bit of epoxy cement have locked the two rear wheels together on the rear axle.
Here's a quick vid of it in action: