All you need to do is edit prototypes/entities/heli_entity.lua, lines 113,114,117
consumption = "3.2MW",
braking_power = "1MW",
weight = 2800;
consumption will modify engine power by increasing or decreasing speed based on fuel usage.
braking_power will modify engine braking power by increasing or decreasing fuel usage.
weight will modify how much the engine has to move, increasing or decreasing both braking and speed equally.
I will try to see about adding settings in game for these features, so editing the files is not required.