There are a few engines that have bulletproof reputations for durability but because of the nature of their long lived mission they aren’t the most powerful engines around. Im thinking along the lines of an AMC 4.0, or A Ford 460. Something completely understressed and overbuilt. Now obviously these are terrible fun engines but they certainly have their place in the market.
Modern engines are, by and large, WAY more reliable than most engines were in the era’s that brought us overbuilt/understressed motors and they are lighter, smaller, more fuel efficient and powerful. That being said they are also much higher strung, and can be quite a bit more costly to repair.
So the question comes: Would you buy an iron pig of engine if it were an option today? Lets give it the benefit of the doubt that you can get ~55 hp/liter out of an engine designed to rev low and last long (consequently it means a nice flat torque curve as well) so a 2.0 would get you 110 hp, but over a broad, low part of the rev range and last forever and be cheap to repair...its also the base engine, so its less expensive than the 1.6T with 160 hp. Would you consider it?
What about in something like a wrangler, would you buy a JK or new JL with the old AMC 4.0 but uprated with crossflow heads, updated fuel management and variable valve timing. it would still only get about 230 hp, which is down considerably to the current 3.6, but last forever and be generally trouble free?
Sure its easy to say yes to old cars with those engines because you want an old ca rto have a reliable engine and they are cheap, but what about new?
Is there even a place anymore for non-commercial users for overbuilt/understressed engines?