Alan, I checked into a muffler from Agco a few years ago, and it was the cheap Stanley at three times the price my local farm supply store charges. Needless to say I turned it down.
I checked into other mufflers, and finally decided to buy a 4020 muffler from John Deere. This required getting a short piece of heavy-wall pipe for an adapter. It was machined on the inside on one end to fit over the MM manifold, then machined on the outside on the top end for the JD muffler. It only cost a few bucks at my local mchine shop, and allowed me to use a much better quality muffler. This muffler is MUCH quieter than the old Stanley. I think the muffler from Deere was only $60.