Even pre-1970, it's hard to think of any combustion engine being "really simple" tbh. They may be more repairable if you have access to spare parts or a machine

Even pre-1970, it's hard to think of any combustion engine being "really simple" tbh. They may be more repairable if you have access to spare parts or a machine shop, but constructing a new engine from scratch (or even a mid-level rebuild) is decidedly non-trivial even if you don't have any electronics at all.