It's nearly trivially easy to build one, though not necessarily cheap. Just replace the battery pack with an engine, generator and fuel. It may be necessary to

It's nearly trivially easy to build one, though not necessarily cheap. Just replace the battery pack with an engine, generator and fuel. It may be necessary to use a turbine engine to meet requirements, hence not cheap, but for flight control you just need to use free open source software firmware (Ardupilot?) that has long been freely available and tweak it.