How much would a 24VDC Electric Motor weigh in order to provide that much output?
What kind of gearing/gearbox/shafting would it require?
What Electric Load would it have to have feeding it....and what would the Generator/Alternator weigh for that?
it would not be 24VDC. More likely higher voltage AC.
800 HP is close to 600 kVA. A 250kVA B787 generator weighs a bit less than 300 lbs IIRC. Motors and generators are similar machines, so figure something like 600 lbs, unless somebody comes up with better technology. You should not need a gearbox for the UH60 tail rotor speed range with an electric motor.
you would need a big honking generator to run that 800 HP motor.