heat exchanger like a 3T-12-2P and a electric water pump https://www.summitracing.com/parts/fss-89026?rrec=true
(just an example) is about all you need. There was a Factory kit that used a double sided Johnson pump
but they proved not great and are now like hen's teeth to find. You need to know that when the thermostat opens the cooling
flow in the block changes direction and that you need a two stage thermostat (the kind with a flat disc on the bottom if you have the later MD2 manifold. If you are going to have a hot water heater
you need to put a 3mm-ish restrictor on one of the two hoses to the heater
. If not you'll overheat the middle of the engine. I have seen this a lot on MD Volvos with "overheating" issues.
My $.02 is Volvo makes good stuff but the MD engine is usually misunderstood and gained an unearned bad reputation, mainly by ignorance of how the cooling system works. I have had zero issues with my MD2 over the 26 years I've had it. Simple to fix. But parts are very expensive and getting hard to come by. But then Volvos don't have the rear seal automatic oiling feature enjoyed by most Perkins