The EPA cares about external noise. The total is capped at so many db at some distance. Obviously everything is making noise but the big contributors are tires and exhaust. I don't see stiffening or strengthening the engine as having a big effect.
However, the motorcycle companies play pretty hard here so they can devote more of the db allocation to exhaust tone than rotating assembly and valve train rattle or gear whine. The Twin Cam and Milwaukee 8 Harley's have a fake timing cover who's purpose is to soak out cam drive noise leaving more space for exhaust noise. I fixed my Twin Cam so I transmit both cam gear whine and loud exhaust which I'm sure far exceed the EPA's miserly limits. But I'm deaf in one ear (exhaust side) and can't hardly hear with the other so with that counter balancer engine I need a lot of noise to hear if it's running.