Hi Tom - Your forklift is a mid-to-late 1980's Komatsu FD40-4. The engine is a model 6D95L-1.
There should be (or have been) an ID plate on the lower part of the front of the "firewall" (the angled upright plate behind the mast), with the Komatsu name and symbol and model number on it.
If that plate is gone, the model and serial number are stamped with a letter/number stamp on the flat part of the chassis, just near to the hydraulic tank breather and your socket set.
These stamped letters/numbers will read something like, "FD-40-4 XXXXXX, with "F" standing for "Forklift", "D" for diesel engine, "40" being the load capacity in "short tons" (i.e. - 4 short tons or 8000lbs or 3630kgs), and "XXXXXX" being the forklift serial number.
The engine should have an aluminium tag riveted to the side of the block (can't recall which side now), as shown in the photo below.