I can't help you on the diameters, other than to say that a good vernier caliper is worth its weight in stainless steel
. It's the number one item to grab before I head
for the hardware
Mufflers can be substituted. Not a problem. In general the diameter will determine the suitability, but be aware that the same size muffler can have different muffling characteristics. You can go from throaty roar to meow in mufflers for the same engine
. My mufflers come in industrial, residental, and hospital grades. There is, of course, a difference in back pressure among them.