Ive heard of people microwaving rubber parts for 15 to 30 seconds to get pliable,but my suggestion is to put on an exhaust cuff and toss the bellows.The cuff is simple and the noise is not that much louder if at all
There are instructions for the $3 (if that) tool in the adults only thread. I used it on mine and it worked great. All the parts can be found at a local hardware store. I had some on hand.
I used a distributor wrench on mine. You know the kind with a 90? bend about 2 inches from the end. Leveraged the bend against the bottom of the exhaust port & pulled the top on & worked my way all the way around & pulled it right on slick as a whistle. I had planned on ordering the right tool but forgot all about it until time to install the bellows. The end of the wrench fit the grooves in the bellows just right with out any danger of puncturing them.