How about an engine family that stayed in front-line service from 1964 through 2006, powering everything from the Rambler American to the AMC Gremlin to the Jeep Cherokee, with a few years of IHC Scouts thrown in for good measure? Starting with the 138-horspower Typhoon 232, the engine evolved into the 199, 258, and 4.0 engines used in just about everything AMC and Jeep made for decade after decade. You can even get one with Renault/Bendix fuel injection (but we don't recommend it). [Wikipedia, Novak Conversions]
My father in law has one of these (or a derivation thereof) sitting in his '96 Jeep. Good motor, pretty rock solid. The Jeep has had issues with the front axle, and the interior is slowly fossilizing, but that 4.0 keeps on ticking over day in and day out.