Progress Rail still manufacture/sell the medium-speed EMD-branded 2-stroke engines (and the new 1010 series 4-stroke engine) - and there is a Tier-4 compliant marine version of the EMD 710 engine (using SCR after-treatment) available, so I guess they could do a rail version of it if there was enough demand.
The problem with using most Cat (and MTU and Cummins) engines is that they are high-speed (1500 - 1800 rpm) engines, which wouldn't be directly compatible with the existing class 56 alternators. The Cat V20 C175 in the EMD US F125 passenger loco is mated with a third-party (non-EMD) alternator for that reason (that loco is a real mixture of suppliers - bodyshell and bogies from Stadler in Valencia, engine from Cat, alternator from Kato, traction inverters from Mitsubishi, motors and traction control from EMD...)