I've been running 71 series engines since I was a boy and am 69 now. I ran them in Vietnam, on commercial boats, yachts and as auxiliary engines on tugs and ships. I've done my own overhauls since 1969. The sleeves are marked and an engine can be rebuilt exactly as they came from the factory. You don't need rocket science.
I love 'em. I consider them to be one of the most reliable engines for marine service ever built. Because they're 2 cycle, you get a power stroke every revolution. They have no injector pump to fail and require no electricity beyond starting. And with hydraulic starting can be started manually. They will run until fuel is exhausted. They get you home. In the military I have run them, out of necessity, in extremely damaged condition, yet they still delivered enough power.
Much of the oil leaking issue comes from the original design 671s using old style seals. Newer engines don't have to leak. They do burn some oil. Probably from the intake sleeve ports at the bottom of the piston movement. I don't consider it excessive.
Parts are available anywhere in the world, even Russia. Many manufacturers make replacement parts besides Detroit. The 671 as designed was able to be rebuilt in primitive conditions including all their accessories, blower, injectors, pumps and so on. I bought my current boat largely because of the 671s. I cruise a 83', 80 boat at 10 knots using about 8.5 gallons an hour. Economic speed is somewhere around 7 knots using about 4 gallons an hour. High speed boats using Detroits do burn a lot of fuel, but so do other engines using similar amounts of hp. Newer electronic controlled engines burn cleaner and use less fuel, but in a lifetime on the water, I have learned everything electric or electronic fails. Usually without warning. People I know running commercial boats with newer engines carry hundred of dollars or more in electronic spares.
As I've said many times, The EPA can take my Detroits when they take the starter buttons from my cold dead hands.