[Ryland]

Honda has some better quality small gas engines like their GX line, as small as 25cc and all the way up to 620cc, the 160cc engine is a 5.5hp, so smaller displacement for the same HP, mostly because of improved combustion chamber shape and the over head valves, the classic B&S lawn mower engine doesn't have an oil pump but instead splashes the oil with hopes of it splashing the parts that need constant lube.
Some of the newer lawn mower engines have oil pumps and oil filters on them, this is a drastic improvement an does a lot to extend the engines life.
If you look at B&S engines, their design has not really changed much in the last 100 years and they have floated along by being the standard.
If you are operating at a single RPM then a simple carburetor works quite well, it's when you start to throttle the engine back that you need to get fancy, keeping the engine temp even, the parts lubed and the inside clean, that is what is going to really help make an engine last, over head valves and quality bearings help it use less fuel too.