Lift started to struggle a bit shifting from Neutral to Forward. Yesterday It just stopped completely. Still works just fine in reverse; lifts, shifts, tilts ect. Kind of new to Forklift repair I have only ever worked on Electric Clarks. Any help would be great, where to start, tech drawings, possible problems. Engine runs great, no other issues.

I have checked/topped off all fluid levels, Tested the switch (lever) at the plug.

V40DSA
Liquid Propane cat engine
I think 80s/90s?