Wow, I had one of those when I was a kid!
I'm working from memory here, so bear with me.
I assume you have the control box and all that too, right? There are two controls, one controls the back-forth of the forklift, the other controls the up-down. Do either of them work? If I recall correctly there's one motor that drives a gear that meshes with the bottom of the fork lift to move it back and forth, and another gear that meshes with the fork itself to move it up and down. Are all these gears meshing properly? It seems to me mine would sometimes slip beyond the gear, I'd have to nudge it to make it mesh again.
You say the motors work, do they spin and nothing happens? Or are they stalled? If stalled, something is binding. If spinning something isn't meshing, maybe a gear is broken or slipping.
Turn the unit upside down and apply power. See which motor is running and where power isn't being transferred.
I hope this helps.
