I'm sure you guys are right about the parasitic losses and tolerances (those were the two points I understand to be related directly to the engine tech), but it looks like a 1976 Chevy Chevette had a 40mpg highway rating w/manual trans. The 2023 Nissan Versa has a 35mpg highway rating w/manual trans. (The Versa's 700lbs heavier, but its engine "flows" more).
100% agree that reducing weight, good tires, etc. all contribute to fuel economy/performance, and I'm sure, though it hasn't been mentioned, that today's technology better serves a wider range of drivers, but I can't see how any of that would contribute significantly to bsfc (the way I drive).
Anyway, wrapping this up (I think), "bsfc" is the idea I was looking for, and I learned a heap. I'm going to put the Lima together with everything standard-- nothing's broke, so no need to replace, but if I come across a set of aluminum pistons for the right price, I'll grab them. Same goes for the cam gear, but if this ever finds its way into a vehicle before I get the cam gear, I'll advance the gear a tooth, ~10 degrees, and if I like it, we'll see...