There is much more to diesel engines meeting emissions standards that just the basic engine. The radiators and their cooling fans, the charge air intercoolers, etc. If you were to take a EMD SD70 locomotive which meets US EPA Tier 1 emissions standards, park it alongside a EMD SD70M-2 locomotive that meets US EPA Tier 2 emissions standards, then take the motor out of the SD70M-2 and install it in the SD70, you would have a SD70 that still would only meet Tier 1 standards. This is because the radiator is not a dual circuit system, and you do not have a large enough charge air cooling system.

Re: the PL42C having permission to run in the US. It did at the time it was built however the standards were raised since then and it would no longer meet the new standards, hence no more can be sold here without reworking the design.

It is a similar situation in Europe, Bombardier replaced the TRAXX 1 bodyshell with the TRAXX 2 bodyshell to meet the tougher collision standards relating to protection of the driver's cabin in a collision. So all the electrical components of the TRAXX 2 will fit inside the TRAXX 1 bodyshell, and the TRAXX 1 is certified to operate in 3 European countries, but no more can be built.