This was salvaged from the side of the road (can't resist council cleanup day) in really good condition. It just needed a new 6v battery. But that's too easy.
I've got a 12v battery sitting around, so let's have a stab at converting it to 12v.
First off we'll need to run the motor at 6v. So using this motor speed control kit we'll take care of that, and get soft start and speed control in the deal. Next -hacking the existing control board.
Basically that's just a bunch of switches and a pair of relays for reversing the current on the motor for forward and reverse, but the relays are 6v coils - can't run those at 12. I'll add in a 7806 to run all the 6v stuff.
To conserve the battery I don't want the motor speed controller running all the time, just when the accelerator is pushed. ONE of the two direction relays operate when that is pushed so I'll just OR those together to operate a large automotive horn relay. That should handle at least 20 amps.
So this is new - I've actually documented what I did, so when it breaks, I won't have to trace it all out again!
Here's the circuits stuffed back into the back of the car. At least the large automotive relay (K1) is screwed down. I should have put the speed controller nearer to the back - the adjustment pot is under the seat and I'll have to pull it all back out again to adjust it. D4, C2 and IC1 are foreground right, hanging precariously off the original control board.