in 2011 I had to go to a "class" I guess you could say, it was more of a pretty technical set of classes that I called "school" (aka training for work). This dealt with lawn equipment and side-by-side UTV's specifically. One of the mowers they built used a little perkins (aka "cat") 3 or 4 cylinder (depending on the HP, one was 28hp the other 30hp). As we all know, emissions laws changed and by 2013/14 "most" engines over 25hp had a DPF. There are different ways to accomplish the same thing but the DPF and EFI was and still is the least expensive way to do it. Note that the EPA (and probably carb) doesn't necessarily mandate specifically the DPF or SCR system, rather it sets a set of rules and laws that dictate how much and what kinds of gases can be emitted from the exhaust system. How the manufacturers chose to meet that set of laws and rules is up to the manufacturer, EPA really doesn't care too much about how, just that it does.
Anyway, on the mowers, the cost of the mower build was to increase about $7500. Of that, $1800 was retrofitting the engine with the correct DPF and DOC which would meet the epa's mandates, and that $1800 would not include retrofitting a reformer system which was about 2 grand total. The rest of it would have been the actual testing and certification process as we were told.
By adding $7500 to that mower, it would have put it WELL beyond what the competitor's comparable mower would sell for, so rather than retrofit, they just quit making them altogether.
Sounds crazy that the state of cali would mandate retrofitting, but I wouldn't put it past them. They are stupid, and they do not care about people. That's why the entire state is so expensive to live in, and from what I am told by the neighbor who moved from there, why many folks are moving out.
Good luck state EPA.