Heck, even a fuel cell would be far better energy/weight ratio, wouldn't it? And since they would be single or few use, you don't have to worry about poisoning the electrodes anywhere near as much.
Under 5kg of batteries, engines basically can't compete. The smallest viable petrol engines (you want engines made in large quantities) are around 4kg and need fuel, and so you end up in a situation in which 4kg of engine and 1kg of fuel is as useful as 5kg of batteries (for motors of that size), but every subsequent 1kg of fuel is then also as useful as 5kg of batteries. But you can't really drop this down much, as a 1kg engine is much less useful than 1kg of batteries, and a drone with 5kg of batteries is really a very large drone.
Drones are additionally typically very small and light, with mass at an absolute premium. A typical quadcopter will weigh under a kilogram and have maybe 200g of batteries for 20-30 minutes endurance. A drone with a 2.5m wingspan will typically have room for maybe 1-2kg of extra payload, and an engine will not fit into the battery slot.
Furthermore, they are extremely sensitive to weight balance issues.
This is to say, once you're in the world where you want chemical fuel, you might as well design a drone for it, rather than trying to retrofit. The mass of retrofitting will mess up your prior drone design, and petrol changes the dynamics of what you're trying to do enough that you might as well just do it all differently.
Essentially, if you're making a 1-10kg drone, you want battery. If you're making a 10-20kg drone, you might want battery, you might want petrol. Above 20kg, you probably want petrol.
1. Scaling. You want to reap the rewards of someone else investing billions, and while billions of ICE engines are built every year, most of them are much bigger than 50cc. 2. Tolerances, as you say. I know for example that jet engines have low efficiency at small sizes due to efficiency being driven by the gaps between certain rotating parts, which are relatively larger. 3. Certain parts that need miniaturisation are more expensive on smaller engines. For example, a 50cc would typically have a carburettor, a bigger engine fuel injection. A fuel injector would be significantly larger per unit. 4. Some parts are just harder to miniaturise. For example, small turbochargers have to work harder and at much higher RPMs to achieve the same boost due to area scaling quadratically with diameter.
Batteries do a lot of things very well such as ramping up and down power output drastically with sub second response times. This allows quadcopter to be very simple mechanically making them scale down really well into a tiny package someone can carry several of them without significant issue. As a bonus tiny drones can be reasonably quiet and cheap.
Aircraft carriers probably are exempt, since intentionally blowing one up as a dirty bomb amounts to a mass suicide attack.
What am I missing?
>internal combustion
Thing with this is that now you need:
a) Fuel tank plus a pump
b) A combustion chamber
c) Gears or a way to mechanically regulate fuel flow
...of which b and c probably need to be metal. For light & small drones you're not necessarily winning here on weight