I've got an 1985 DT-60 and a 1997 DT-85, 2 stroke oil-injected. Both have similar issues, probably fuel system issues, and I am having trouble finding shops that are willing to work on them. I'm bailing on these engines and going with Johnson-Evinrude V4 in place of the DT-85. If you have good local support, the Suzukis are great when they're running. The 1985 has serious parts availability issues as well.
For just one cylinder... others have suggested bore out only the one cylinder, with new rings and piston, and new rings only in the other cylinders. Is the 55 a 3 cylinder like my DT60? Of course, the other issue is *what cause(s) led to the low compression in the one cylinder?* Cooling issues? blocked water passages? I'd sure want to have a good idea of what led to the one cylinder issue and know that the cause was being addressed in the rebuild so that it won't recur 6 months down the road after spending a ton on a rebuild of an old motor.