are you thinking of buying an old used fridge and pulling the cooling unit off and putting it on yours? wow thats some crazy thinking.
firstr off you are buying a used fridge in unknowna condition, anyone can say it was running good when they took it out, plus when the heating units go they were usaly good one day then the next day they aren't that great. so how long is that used fridge going to last in your unit, how many times did they run it out of level? are there already crystals forming in the tubes? did they just take a dead one and flip it upside down to move the crystals so it will work for another month or two? so many unknowens, then there is the work of swaping them over if they are compatable, generaly they are not. you would have to go to a site that sells them and look up the two models and see if the replacment heat exchager is the same part number and measurments.
I know up here its about 800 bucks for a new exchanger , about 2K for a brand new fridge and anything worh buying as a used fridge that I would put in my rv would be a 1000-1500 unit, to much work to take a chance on a cheepy. and a new exchange basicly turns your 30 year old fridge into a new born, aside from the motherboard and usaly they cool better than when they were new.