Even a 3K G83P had the slant 6. I think they called it a ramp climber, and it could certainly climb a ramp, even at 1/4 throttle.
I had to put 2 pistons, bearings , and gaskets in a TM Mazda a couple years ago. The pistons were $400 each without rings, oil pan gasket $100, head gasket $100, etc.
Keep in mind a slant 6 operates at a much higher RPM than a diesel and I would think a diesel forklift is geared for a slower turning motor. You will also have to figure out how to connect the flywheel to the torque converter.
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