We just went through this replacement process a few months ago.  If you're willing to spend the money, probably the best option is to have a new one fabricated in metal or fiberglass so you can continue to use it as a step.  I wasn't able to find anything off the shelf that would work for that, and in the interests of time and money, we went with a Vetus.  Specifically, we ordered a model LSG75 and then added the SET0004 connection (because the two connections are different sizes on our boat, but the mufflers all come with the same size on both ends).  I think one could just as easily order an LSG90 with a SET0003 and swap the connections as appropriate, but Defender has the former combination in stock and doesn't stock the latter.