Correct.

Both Slim Line and Bisson make pitts mufflers for the 61 and the 91 so be sure you use the one for the 91.

Tatone and J'tec make a contraption that can be plumbed up as a in-cowl muffler but they have even less volume than your present muffler.

One of the problems is that the 61 mufflers will bolt right up to a 91 and appear to fit perfectly.

Anymore, no one buys the 61 engines, everyone uses the 91's.

Enjoy,

Jim