DC motor power units don't have a fixed gpm output like engine driven pumps. The motor rpm and therefore gpm drop off dramatically with increasing pressure load. There is a tradeoff with pump size for more flow or more pressure, but not both. (Unfortunately I am not aware of any DC power units with tiny two stage unloading pumps. There just isn't enough hp in these small units to make it work, but it is a nice dream.....)
as noted by others, to increase force you need to increase cylinder size or pressure or both.
Assuming the package was designed at maximum lifting for minimum cost, there may not be much extra in the design. then I would be concerned that upsizing the cylinder or pressure makes the next weakest link the frame or structural parts.
let's take different tack: I would not try to increase capacity beyond its original design, only make sure it is back to the flow and pressure it was designed to do Possibly the relief valve in the unit is malfunctioned or set too low. Possibly the pump and cylinder were mismatched at some replacement time in the past.
If you are smoking wires or motor, then it is probably overloading the unit, or the wiring is too small.
I would install a pressure gauge in the cylinder closed end line, a voltmeter on the electrical terminal right into the motor, and an ammeter on the line to the motor.
See what the lift pressure, the voltage at the motor, and the current draw is when you are lifting.
If pressure is low, then solving that could restore your lift power, NOT increase it from the original design, just get it back up to what it should be. I'd guess the rated pressure is pretty low with the cheap single acting type of cylinder and DC pump. This is not a commercial dump truck quality level I assume. So maybe somewhere around 1500 psi or less.
If terminal voltage is too low, the wiring may be undersized and creating too much voltage drop. If amp draw is too high, the unit may be mismatched to the job.
Those are the cheap and easy things to check and to maybe fix.
Otherwise, maybe the whole truck/lift pump/hoist/brakes package is doing what it should do, it may just be too small for the loads you need to haul.
kcj