The first experiment you can do is to see how much power the LPG on/off valve takes. You don't want your engine sitting there leaking LPG ? Do you stop the engine by shorting out the ignition?, because the ignition must be switched on, then the starter can be started and switched off when the engine is running.
One thing that is very important is to check that the starter cables are good and thick and well connected, because you are going to try to run the starter when the battery is running down, so if it does not fire it will still turn over and you end up with a dead flat battery.
The circuit is fairly trivial once the above details have been sorted out.
Frank