I might be missing it, but the only reference I see to Kubota engines is the 2.5L optional engine on the 4000-7000 lb trucks.

I did just receive some addition information from Yale. Still nothing stating what the base engine is, but this might help:
4-cylinder single overhead cam design with 4 valves per cylinder
Cast iron engine block with a five main-bearing design for added strength
Aluminum cylinder head

I don't find anything matching this in Kubota's engine line-up.