<Larry? Can a single or twin-engined prop aircraft be 'hot-wired' to start the engine(s) up by someone who illegally boards a private plane?

Yes, it's easier to far hot-wire a plane than a car, especially if you're talking about a piston-engined plane. It's damned near trivial. All you have to do is cut the grounding wires to the magnetos. Any juvenile car thief could do the job in seconds. Even the door locks on 99% of private planes are a joke compared to car locks.

Larry? I strongly suggest that you get a handgun permit, buy a 9.0mm
automatic, and a Kevlar vest, so that you will have a measure of protection should you see some [viscerally ominous] dark or olive-skinned stranger approaching your private-aircraft with a suspicious "package" or "object". Larry? You're back in the armed-forces, so-to-speak.

Not to worry. I do a very thorough preflight inspection before every flight. There's no way a package would be in my plane and escape notice, especially not a package containing enough explosives to make my Cessna into a useful suicide plane. I fly solo 99% of the time (not that I'm antisocial, my wife just doesn't care very much for flying), so being hijacked really isn't very likely.

A 150 mph, one-ton Cessna, may not be as dangerous as a 600 mph, 50 ton jetliner, BUT!...IMAGINE what a crashing Cessna, or a Piper-Cub,
with a nearly-full fueltank {and maybe a full jerrycan to boot; and possibly a couple of railroad flares loaded onboard as well} can do if it crashes into Denver's, or Buffalo's, 40,000 seat packed football stadium!...Horribly, tragically, no longer an unthinkable scenario..

It's really not such an unthinkable scenario. There was a movie back in the 1970s about a terrorist plot to crash a anti-personnel bomb-laden Goodyear blimp into the Super Bowl ("Black Sunday" or something like that). It wouldn't surprise me in the least for future sporting events to feature vastly stepped up security, perhaps including some military or police personnel armed with Stingers. It also won't surprise me in the least for temporary flight restrictions to be enacted within 2 miles of a major stadium during a sporting event or concert. A stadium filled with thousands of people makes for a very good target. An explosives laden private plane could kill a lot of people and would be trivial to do for a suicide pilot.