There should be a data tag on the unit somewhere. It may have, at least, a machine serial number, which you should be able to derive the engine type from that. If there is no tag, check for a stamping in the frame around the right front wheel/frame rail. I'm not 100% sure on the older 3C models, but on certain machines they did stamp the machine serial number in the frame.
I do know that VERY early backhoes used Leyland engines, but most are Perkins. It's just finding out what series from there. Good luck.