The problem isn't the generator itself, it's the fuel it needs and the vapour left in it's tank after it has been drained.
Why do you need heating in the first place?
Is a portable petrol genny enough to heat an aeroplane? It would have to be a small plane to have much effect, so the genny is going to be a significant weight penalty.
You can't safely or legally carry the genny anyway, let alone it's fuel cans in the aeroplane, and draining the genny's tank may make the risk worse. Totally unacceptable.
What airport would allow you to leave a petrol genny running close to an aeroplane?
The insurance company, to say nothing of the CAA would take a very dim view of this indeed.
Forget it. Buy a down parka instead.