On a petrol engine I'd guess the carburettor would be the main sticking point - compared to modern fuel injection which can adjust instantly if the emissions aren't right, carbs were actually pretty bad at providing accurate fuelling across all situations.
I remember a warranty mod on 1.1 Metros (SU carb, I think ?) which involved adding a tiny external damper to the throttle linkage to stop the engine revs dying if you took your foot off the accelerator quickly. Very 'agricultural' really and didn't actually fix the design fault, hiding it instead
We probably don't appreciate just how good modern ECU controlled injection is nowadays.