Wayne,

First Caveat: I've never worked on any Detroit Diesel engine (that
I can remember)

Wayne.B wrote:

Is there any way to measure compression without removing injectors or
other major components on DD671s?

Don't know (shrug)

Second question:

Since the DD671 is a 2 cycle engine, I'm assuming there are no moving
valves, just fixed intake and exhaust ports in the cylinder walls.

There are exhaust valve/valves in each cylinder in a two-stroke
turbo-diesel. See http://auto.howstuffworks.com/diesel-two-stroke1.htm

Also see http://www.marineparts.com/partspages/DETROIT/DET12.HTM for a
list of exhaust valve part numbers including for the 671.

If so, I'm also assuming that any loss of compression would have to be
from worn rings and/or cylinder bore. Also correct?

See above. Loss of compression could be due to rings/cylinder, exhaust
valve, etc.

Good luck with it.

Don W.