Electric is not as realistic on a budget as a gas powered one would be. It's going to be hard work to make one that works. You can take an old lawn mower, take the tire off the rim or mount a bare rim that fits. You'll have to position the mower/tractor first and block up the rear so it wont move. Also you'll have to secure it in the ground a bit so it wont move. You also will want a second pulley on that end to add tension so it wont slip when you are on the rope. Then at the far end you'll have to drive another post into the ground and put another bare rim on that side that can spin. To do this right, you'd also need to have tall posts with rims that carry the rope back downhill above your head, but I dont think it's that phesable with just a kid and a dream.
TLDR: Just hike and enjoy the exercise unless you're really committed