The rear unit in my 2003 44AC is a 9k BTU Taylor Made. I haven't replaced it since it still works perfectly, but there are great, inverter-throttled options that are more efficient and have the ability to throttle down to lower the output and keep the set temperature instead of constantly cycling on/off (which I kind of detest as a liveaboard).
Inverter A/C's:
VELAIR
There's also DC A/C's available now that are top dollar. But offer the best efficiency if running off batteries.
Personally, the inverter A/C's are the most promising/interesting to me.
The existing unit lives in a box in the starboard side rear lazarette. I'm 5'-11", 200 lbs and can climb right into the locker. My boat also has a panel in the aft cabin that appears to come off to provide access to the unit from the other side. I would start by getting access and taking measurements of the footprint and height to compare to modern units.