I am not sure if you have handled this already but I will give you my advise. The prima 50 heat exchanger was manufactured in 2 different sizes. The earlier engines came with a tube that was 2 inchs in diameter and after around 1995 they swiched to a 2 1/8 diameter tube. Both tubes will work in ether engine but the sleeve is differnt for the heat exchanger depending on the one you have. The larger one is made of ruber while the smaller diameter one is hard plastic. If you have the larger one made of rubber, when you remove the heat exchager it will more than likely fall apart and need to be replaced with a new one. The smaller hard plastic one will probably be intact but make it very difficult to remove the heat exchanger from the rear. You may have to push the heat exchanger forward fair enough to get the plastic sleeve off then remove the heat exchanger from the rear if that is your only option. The other option if you do not have enough clearance forward or aft is to remove the whole manifold then remove the heat exchanger. I hope this helps. I have worked on a lot of these engines and they are prone to over heating
if the heat exhanger is dirty, of course you can do a fairly good job of cleaning the exchanger with it in the engine by running a small rod through the tubes with the ruber caps off, and then using a good antifreeze
flush and flush the engine multiple times with clean water
. Pay close attention to the ends of the heat exchanger, where the sodder holds the small tubes in place. Once these sodder joints fail, you have to replace the whole heat exchanger, which if you need one buy from Orcamarrine.com for a new one for less then half the cost of a new one from Perkins.