The Alco Fa-1's were rated at 1500 or 1600 HP, depending on when they were built.  For use as Head End Power, the LIRR removed the traction motors and reduced the horse power (de-tuned the engines) because they didn't need that much HP to run the generator for lighting, heating or cooling the passenger cars. These are the "untouched" engines in your email excerpt. 

In 1978, the LIRR purchased engine-generator sets from Detroit Diesel. They consisted of a small diesel engine, a generator and a control panel all mounted on a single steel frame.  The original Alco engine and generator was removed from the FA body and the Detroit Diesel "sled" dropped in.  The sleds were fuel efficient and quiet running.  Info: Steve Rothaug

Detroit Diesel sled being prep-ed for installation in the first F7 converted for HEP use. Morris Park. Photo: Steve Rothaug

The FA's that were converted over had Detroit Sleds placed in 1983. Units that had the first HEP modification, but untouched were:

607
609 (scrapped in 1993)
611
612 (destroyed, and scrapped in 1993 due to a fire)
613
614
616
Units that had HEP modification were: Also, those 2 FA's were the first ones to have the quieter HEP's.

615
617

The quieter sleds were in fact placed in 1987. 605, and 606 were totally rebuilt to the point that where the original engine room doors, and steps were, were taken out, and relocated. c.1983-84, or so, such FA's as 605, 606 had the quieter engine.

In the late 80's early 90's however, 615, and 617 had quieter engines installed, or maybe mufflers because besides that, there was extra grilling placed in front of the existing grille that was left for exhaust when the HEP's were converted.  605 and 606 had the original grilles around the headlight and original headlights removed, and plated over using the vertical headlight in its place. Research: Neil Feldman