Don't understand reference to mandatory CAT 2 implements.?? Simply use a reduced bushing. My MX is cat2, before purchasing Pat's quick hitch, I just used the bushings. CAT 1 implements work fine using The bushing.
The DOC catalyst emissions must be developed and tested by each individual Engine producer. This is an expensive process. The DOC uses more aggressive EGR which some folks feel is harmful to cylinder liners. DPF is the easier path to compliance.
RK did not offer HST on their 55 hp open station but they have now seen the light, revised the specification. Massey will also if demand is demonstrated.
Deere Utility R series and up, Bobcat Corp, Mahindra also use DOC now, instead of DPF. Maybe others?