putting on a bigger muffler isn't going to help. most of the noise is coming from the block and without a water jacket there to absorb the noise it is going to sound like your lawn mower. they wern't made for trolling but to power small rafts and canoes so you didn't have to row. and putting any kind of cover over it would restrict the air flow to cool it. I don't know if the noise will scare the fish, but ear plugs my be your answer untill you can find a different motor. dfly