I long for the "good 'ol days" when a 6/1 could be imported for a few hundred bucks.
However, the reality is, Listeroids and their variants have gone way up in price. Listeroids are currently priced similarly to modern, emissions compliant engines, when compared per HP.
Certainly, a well designed modern engine can be a better choice for many people that have significant power needs. In my case, my 20/2 is nowhere near as reliable as a modern 20HP diesel. Nor is it as efficient.
The advantages of these old engines might be ease of maintenance, robust construction, hand crank, and superb fuel consumption under low load conditions. It's not unusual to see a 6/1 configured to consume about a pint per hour under low loads and at low RPM.
This is where the 6/1 and 8/1 make sense (at least to me) . Few commercial diesels are that low in HP, and therefore, the commercial products burn more fuel under low load conditions.