Moped Army

It depends on the design, I would venture that the most durable engines are the ones with iron cylinder barrels, even if a bearing blows up, the piston seizes, or a ring cracks, the worst you have to do is bore it out. So the PA50s are iron, so are some Garellis, along with Laura M48s and the Velosolex engines. I would guess that an M48 would be near the top, not a fast engine but very simple, the PA50 Honda is also very good. On the Velosolex the engine is fine but the internal ignition coil is an Achilles heel.
There aren't too many bad engines with the name brand machines, the longest stock moped ride was on a 1978 50V Mobylette in 1978 by a fellow named Walter Muma. 11,000+ miles in 1978 without anything other then the odd carb cleaning, plug changes, and decarboning thanks to the 70's two stroke oil.
There isn't such a thing as a no maintenance moped, chains need oil and bearings need an occasional greasing. There is a an engineering joke about what no maintenance and low maintenance mean. No maintenance means impossible to repair. Low maintenance means nearly impossible to repair.