The regulator for a butane can would probably be heavy, not to mention how dangerous that would be if your engine quit on you out over some trees or a distant field. You would need an ignitor source too. The weight penalties keep going up as one thinks about it. The fire risk is enough to make this a bad idea, don't you think?
I was basing my idea off these.
they are small, lightweight, and you can control the ignition.