Just search "drone engine" or "UAV engine", there are tons of fuel-powered engines (many optimized for JP-8 since the military likes to be single-fuel) made for planes of various sizes.
The larger ones tend to have an electric generator to power the control surfaces, smaller drones tend to carry a battery for that and use the engine solely for forward propulsion.
This is for fixed-wing only, so it would apply to Zipline P1 but not P2, by the way. There have been some engine-over-electric setups to run multirotors from a single powerplant, but it's generally awful. It may or may not work for hybrid architectures like a quadplane, or a single-engine-VTOL like a tailsitter, all these things are in active development.
The larger ones tend to have an electric generator to power the control surfaces, smaller drones tend to carry a battery for that and use the engine solely for forward propulsion.
This is for fixed-wing only, so it would apply to Zipline P1 but not P2, by the way. There have been some engine-over-electric setups to run multirotors from a single powerplant, but it's generally awful. It may or may not work for hybrid architectures like a quadplane, or a single-engine-VTOL like a tailsitter, all these things are in active development.