The simple way to stop this is to fit a tap in the oil feed. Kingpin components sell a very nice one that has an electrical switch in it too. They also sell a magneto end cap with an external contact in it. With this wired to the switch the engine cannot be started with the oil turned off. My brother and my son both have these on their G3L's and they work very well. I rebuilt my own G3L with new old stock crankcases and oil pump shaft so it doesn't wet sump anyway.
If the bike doesn't do many miles and otherwise runs well, the tap and switch are probably the easiest and cheapest option. The rebuild I did on my engine was not cheap, it was also quite time consuming...