My two cents.
Engines are inanimate objects even if they drive you to the stage that you think they have it in for you. The temptation can be to get rid of the thing before it drives you mad. But just like health issues proper diagnosis and treatment should work unless the hours are just too high or there's been terrible maintenance or catastrophic failure.
A repaired engine "should" be just as good as a new one....even new ones have problems. I have 2 new 650HP Yanmars on a boat this year. One of them had a faulty fuel return line initially diagnosed by a noise at idle coming from cylinder 5. Recalibrating the fuel pump, inspecting the valves and piston etc all occured before the problem was identidied and the engine was fixed. And it was brand new.
Don't underestimate the complexity of changing the brand of engine - hidden problems and costs. Out of the frying pan and into the fire....
I'm a Yanmar fan and but some Yanmars engines made in Europe, some Volvos in Japan etc... not that much difference in brands to warrant the complexity of changing. The Amel 54 is an awesome boat. Volvo has long been standard in top boats, Swan, Hallberg-Rassy and many others. Jamie's been unlucky but only he has the full picture..