I think this is one area that we can learn a bit from.
I was just discussing the issue of a broken heat exchanger with Jill.
What I might try is to do is open the heat exchanger and take the core out. How hard is it to do that on a Volvo? It's easy on our Yanmar. Once out, I could find the bad tubes, then plug the bad ones. Once replaced, you might not be able to run at full power, but you could still get moving.
Just an idea.
Greg