I have never had one of those throttles apart so not sure how the spring sits inside of it.
The controller's throttle connector wires can be tested to determine which throttle wire they go to. The black wire should be negative and one of the red wires will be around +5 Volts and the other will be +36 Volts (or the same Voltage as the battery pack). The +5 Volt wire goes to the +5 Volt input to throttle wire and the +36 Volt wire goes to the battery indicator positive wire. Once the functions of the red wires have been determined then it can be concluded that the green wire goes to the throttle signal output wire.
Here is a wiring pinout for the 6-wire Dune Buggy throttle.
You are welcome. The original 24 Volt throttle will work with the 36 Volt controller as long as the 24 Volt throttle's indicator wire is not connected to the 36 Volt controller.
If you want to switch over to a thumb throttle with a 36 Volt indicator so you can monitor the battery level then our THR-68 would work for that and it is available on our 36 Volt Throttles page.