if it was me, (and I have looked at a couple of electric forklifts) I would start with knowing the battery is good.
Do this by doing a "load test", where you read (I prefer a digital Volt meter) the voltage of the battery 'at rest' and then tilt back or (best) to run the forks to their max height, then read the battery voltage while the hydraulics are at "stall" [it takes more current to lift than to drive and it is sort of dangerous to try and read a meter while the machine is travelling]. if you have more than a 10% voltage drop between 'at rest' and 'hydraulic stall', you will need to get the battery up to stuff first. Do NOT stall the motor more than 5 seconds, it should only take 3 seconds to get a good reading.