Hello, James - There are no easy answers here. The engine you have is an M4-121, in Hyster-speak. Conversion to a GM 4-181 (3L iron duke) wouldn't be easy or straightforward either since the hydraulic pump would be driven at the flywheel end of the engine from a chain drive through a wet bellhousing - incompatible with your setup and existing plumbing/pumps. I'd suggest finding a decent used engine through a used forklift parts supplier - it'll be the most economical and provide the best chance of success overall. Here's a list of the companies I'm aware of - I hope one of them can help, but you'll need to have your model and serial number handy for them to reference:

The Berns Company
www.thebernscompany.com
(562) 437-0471

Morrison Used Parts
www.morrison-ind.com
(616) 225-8670

Material Handling Wholesale Corp
www.mhwc.com
(800) 634-0350

Martin Industrial Truck
www.mit1976.com
(219) 362-8500

Lift Parts of Oregon
https://liftpartsoforegon.com
(800) 584-9119

Atlanta Liftruck Salvage
www.atlantaliftruck.com
(800) 999-9866