A few years ago, a Cessna 182 burned up when a Weed Wacker tipped over and spilled fuel. Luckily, it happened during taxi so the pilot was able to jump out! People often forget that batteries can be just as dangerous. A small puncture and exposure to air is enough to cause many batteries to burn aggressively. People need to take reasonable precaution when hauling scooters and such. In addition to fire safety, they also need to be properly secured during a 10g emergency landing condition.
I have several layers of redundant fuel protection when I carry my motorcycle: 1) The motorcycle has been extensively modified to prevent fuel spills. 2) If fuel were to escape past multiple check valves, it would be captured by a backup catch can. 3) The entire upper surface of the motorcycle loading system forms a sealed spill pan which is covered with enough fuel absorbent material to contain 110% of the motorcycle liquids. At this point, I'm far more concerned about the fuel tank in my lap when flying older airplanes.
CFR 175 contains hazmat rules. Most of the regulations apply to goods in commerce and not to personal flights. However, even when they aren't legally applicable, the rules still contain excellent safety advice for anyone thinking about carrying a gas or electric vehicle.