I think I can understand why they would say that. Technology over the past 20 years has greatly changed. If the break point on cost is within budget I'd go with a new engine and gain the rather considerable savings in weight and fuel economy and count the controls and monitoring capabilities as gravy.
You might be able to find plenty of spare parts for that old Ford (for example) but can you change the impeller in 5 minutes like I can on my new Westerbeke that I've fitted with a swap in thumbscrew quick change unit?
Can you plug it into a NMEA 2000 network and monitor it on a laptop for compression, fuel flow, power, temp etc - that really lets you know what's going on inside it - or do you troubleshoot problems by ear and smell?