This site has a ink to the Volvo parts site. http://www.volvopentastore.com/
They have your part. $1300 (ouch)
E-bay has some listed too.
One new for $1000 a used one for $225
Is your core so bad it can't be acid dipped?
If you buy used make sure it's not a salt water unit,
If your overheating it's more than likely the manifold????
Honestly I don't know. I'm new to the boating world. This is my first boat I ever bought. It looks like the heater exchange leaks from under the unit. Looks like a gasket and 2 plugs. I removed one of the plugs, but the other one is on pretty tight. And also a copper tube is loose. If that makes any sense.
The pipe probably isn't bad but just needs gaskets/seals.
Volvo uses a special o-ring/seal
The 2 screws, both are drain plugs.
One is for the heat exchanger the other is for the reservoir.(I think)
Spray with WD or PB Blaster(best) let set.
Small propane torch and slowly work screws out.
The bolts/screws will come out just don't force anything.
This site can lead to a Volvo parts site.
Look at the top to find the parts links.