If you want to build a 6 kg machine, you should have a look at the German Raptor rules.

But to make the search shorter.
No brushless ,no brushed larger than speed 700 size and no rare earth magnet motors allowed.
12V SLA, 10S NiMH, 3S LiPo or 4S LiFePo4 batteries.
Voltage boosters or extra batteries allowed for valve control.

The rest is classic featherweight rules.