With a bit more reading it's obvious that some US operators have been or are rebuilding just about anything with 4000 odd horsepower preferably with split cooling .
The big issues with the EMDs seems to be cooling capacity EFI and Siemens electrical gear in the SD60/70/90MACs .
The Siemens inverters etc are drying up so the logical solution is to convert to the Mitsubishi parts like the SD70ACe series got .
The 60s and 70s don't seem to be getting the ACe sized cooling group but are stepping up to Tier 0 or 1 emissions standards .
Canadian built 90 classes look very much like a lightweight SD60 on paper .
I think the flurry of rebuilds/upgrades was driven by perceived fears of the Tier 4 units compliance issues and the possibility of them being more expensive to feed . I haven't read any reports on how the 70ACE T4 goes but someone will eventually post their opinions .