PSI had been doing the 3.0 and 4.3 GM engines for them for a while and GFI engine division, which is now owned by Impco, doing the 2.4 GM. So it does not surprise me that they would let PSI do the Kubota engines as well. Basically PSI dresses the engines to that OEM's spec. mostly the fuel system.
If memory serves me right it is who ever holds the EPA certification.