Look on the exhaust manifold side of the motor between #2 & #3 cylinders about half way down on the block, there will be a stamping of engine model & serial number, should be a 4G52 Mitsubishi motor. Do you see a stamping on the data plate that reads F17A-00497?? If you see that on the truck,that should be the serial number. There was a FG20 built in the mid to late 80's just as you described, check out Service manual #99719-01170a to confirm,this manual is for Chassis & transmission,engine is covered by a different manual. You may also want to look around eBay for manuals,I find quite a few out of print ones on there from guy's getting out of the forklift field.