I read a book years ago that described this idea very well.
I'll try & do my best...
Ok, they used a small transistor radio as the reciever.
& a small transmitter made from a Radio Shack store.
The transmitter is the sort they call a 'Bug', a couple of transistors & resistors etc on a board, cost about $10 each.
The idea is that you tune the radio to a part of the band wheres theres plenty of static.
Then tune the 'Bug' to transmit to the radio at that same frequency.
Now connect a relay to where the speaker is on the radio & connect the other end of the relay to the kill switch.
(you are gonna have to figure that bit out yourself, sorry)
Yeah, thats about it.
How it works?
Well, while the transmitter is transmitting, the radio will be silent, no static, because theres no power comming out of the speaker while its silent the relay won't trip.
But when you turn the transmitter off, or the reciever gets out of range, the radio will now pick up static, that static is enough electricity to trigger the relay & it will kill the engine.
Yeah, thats it.
I figured the idea is a good one as it can be fitted to the gocart & if someone (there always is..) won't get off the Gocart, just turn the bug off & the gocart dies.
Now they have to push it back to you.... haha.
Also, if its stolen, it won't work untill they figure it out.
& if someone drives the gokart out of the range of the transmitter (your specified driving area) it dies & they have to push it back to you again.