Wouldn't need to, as the EPA rules only apply to engines manufactured after 1/1/1973, and remanufacture of engines pre-dating 1/1/1973 only
if that reman upgrades the engine to something different than original spec. Therefore a Budd could have been remanufactured multiple times to like-new condition now containing more new parts than old, but since (1) it was originally built before 1973 and (2) provided that the Detroit engines have been maintained to factory performance through all those remanufactures...it's fully exempt.
Doesn't matter if it's an RDC, a first-gen GP40, or a 2-8-2 steamer...if it was built before Watergate, and you keep rebuilding it without changing the type
of main engine make or that engine's performance rating, you can operate wholly outside the EPA tiers in perpetuity under the most-current 2008 regs.
https://www.ecfr.gov/cgi-bin/text-idx?S ... 3&rgn=div5
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^-- Can jump to the relevant passages just by doing a CTRL-F browser search for "1973" and clicking through the dozen or so search results.
If you do repower it with something different than factory spec then, yes, it qualifies under the new rules and must conform to the emissions tier for the given model year of the upgrade
remanufacture. Which would mean Tier 0 for anything 1973-2001, as I very much doubt there's been a single RDC yet re-powered with a 21st century engine make.
The fact that nearly all operable Budds escape the EPA regs is why they're so sought-after. ADA isn't hard to mod, and anything newer than a Tom Thumb can be hooked up to PTC with the right interface for the make/model's controls. Since that very Budd-like belch of smoke is a bit of a turn-off for modern uses outside of museum ops, the most you'd be obliged to do is install sensible add-on emissions controls to tame the visible and smellable exhaust at the stacks. Something that can easily be implemented by many different means without modding the engines themselves.