My Catalina 36 is 22 years old.  I was frustrated last summer, then decided our equipment is just OLD & TIRED.  So I bought replacement Adler Barber (the ability to use same mounting bolts was HUGE) compressor & cold plate for USD 1700 from Defender.  Installed it with help of one friend and refrigerator is working great +3 to +4 C (36-41 F) and very satisfied.  The first cooling cycle will take some time because of the heat retained in the insulation.  Six weeks later we've had a super hot week and defrosted cold plate.  Ah, dumped a bunch of ice coating the cold plate and we have super cold beer (and milk).  Sometimes you just have to bite the bullet & spend the bucks....

I just replaced mine. If you can travel down to Seattle either Fisheries or Sure Marine.  It's an AB CU100.  I bought it at boat show prices.  Fisheries routinely gives discounts if you ask.  

I agree with Peter.  A couple of yearsago, after researching various systems, I replaced our system with a new Adler cold machine. It was an easy switch out, same for same.  Much more efficient running.

You will definitely need a second person to help, especially for feeding the copper tubing from refrigerator box to compressor mount area.  Have extra wrenches as the instructions were wrong on wrench size to hook up copper line to compressor.  Recoil the excess copper tubing gently, to about 12-15 inch diameter and then hook up to compressor. Don't bolt in new compressor until the copper tubing is connected to compressor and excess coiled tubing is under the compressor mounting shelf.  A second set of ribs will be useful as the installer has to contortion body to install cold plate in refrigerator box and hanging excess copper coil in aft locker.  That
last step is a pain in the ribs.  

Do replace both cold plate and compressor.  Yes, expensive but well worth the effort.  

One of the best maintenance tips I can give you on these units is that if the cold plate starts to develop ice, the cooling efficiency will drop dramatically therefore DEFROSTING is needed. Spend the 20-30 minutes defrosting the cold plate and your unit will be nice and cold again.
Buy a $10-$15 indoor outdoor / thermometer that will allow you to monitor the temperature of the refrigerator with out opening refrigerator.

I have never used my fridge. Only recently I was curious if it worked. After it ran for 2 hours non-stop the temperature was 42F and not dropping. How long does it take to reach 30F on your working fridge?

You will need at least 24 hours if not more to reach a stable, acceptable temperature. The insulation of the box is not very good.  I have lined our box with over an inch of additional insulation.  When running the refrigerator it helps to have something in the box to cool and help keep it cool, such as.gallons of water.  If you freeze them initially, it helps to cool the box down.

Haro, after starting up my new compressor & cold plate the first time, it took my refrigerator about 12 or more hours to get to 38-40 F degrees.  Remember the insulation keeps heat and has to shed that heat.  The lowest temperature my new system has reached is 34-35 degrees.  I doubt yours will ever reach 30 F.
On super hot days we do throw in some frozen water jugs to help keep temps down.   On days we have many visitors we keep a cooler with iced beverages in the cock pit, avoiding opening & closing refrigerator.

We have an issue with semantics.  The large compartment with the rectangular box up top is the refrigerator.  The rectangular box up top where one would make ice is the freezer or "cold plate".  It is this area that provides the cold for the refrigerator.  So the cold plate will be below freezing (32F = 0C).  I put my remote temperature sensor at about the furthest distance away from the cold plate and that is where I measure the working temperature of my refrigerator (at best 34-36 F, usually 2-3 degrees warmer).  I hope my explanation is of help to you. 

Haro, we keep a couple of trays of ice in the freezer, sometimes 3. But it needs to be colder than 32 degrees. Play with the thermostat settings and get it down to about 28.  Also, a helpful hint  If you prefreeze "flat" food, you can store it in the shelf under the freezer compartment, even along the one side.