I'm not sure about Kubota models. But a direct bolt on compressor is an option on some Deere engines pre tier 4. Not sure about the tier 4 engines. Compressor mounted to a plate on the side of the crank case and ran off the drive gears. Just had to remove a blank plate and add the compressor and gear. Compressor also required a couple of coolant lines, and oil pressure line.
It was an expensive add on though at around $1,800 for the compressor, gear not included. Coolant lines ran a couple hundred each, oil line was around $50. Then you still needed to run the air lines, tank and pressure reliefs, which the fittings will nickel and dime you to death.
If you could figure out a spot to mount it up front using a longer fan belt, you could mount one that's belt drive. But they do take up space and would be outside the hood and could very well have clearance issues
I had some equipment at work that had the belt drive compressor.
Had one that the pulley snapped on. Did a number search on make and model. Come to find out, one of the main applications was on old ford trucks.