Actually most of the emission are a result of the valveless design of the typical two-stroke. Small carb 4-strokes have the same issues you describe but have mu

Actually most of the emission are a result of the valveless design of the typical two-stroke. Small carb 4-strokes have the same issues you describe but have much better emissions due to the valving, but lacking a cat they will never come close to and automobile as far as emissions go. Seals wearing out will cause lower compression and more blow-by reducing power and increasing emissions but probably not as much as a valveless two-stroke which simply has intake and exhaust ports full open during part of the cycle.