OK, I'm going to throw a spanner in the works here and say that a 2 stroke (not a 4 stroke) glow engine can be run without a prop or a flywheel but only under certain circumstances and I don't recommend anyone try it. It's already been said that sometimes if you break both blades off a prop then the engine will keep running but at extremely high revs. That's called a shaft run and can quickly break things in an engine. The time I had one it split the disc valve and loosened the screws holding the crankshaft housing to the crankcase. Without a flywheel the engine can only keep running if the revs are high enough for the crankshaft to act as a flywheel and have enough angular momentum to turn the engine through compression, the same as a flywheel can do at much lower revs. This is probably in the area of 15,000 revs or so.

Way back in the dim dark ages some racing (unthrottled) engines were run in by starting them with an old wood prop, tuning the mixture extremely rich then shearing off the prop blades with a broom handle. Well I said don't try this at home . The revs would instantly jump very high but the peak revs were then controlled by richening the mixture even more if necessary. The engine would stay at a safe rev and wouldn't overheat because the very rich mixture let the fuel do all the cooling on the inside.

In effect this is how some of the car guys are starting to run in their engines. They start them as normal at idle then slowly open the throttle until the wheels start spinning then richen the mixture to lower the revs, open the throttle a bit more, richen etc until they're at full throttle but with the engine so rich it can't rev very high. That's also a shaft run but it can be done at lower revs because of the heavy flywheel.