Tractor Supply has a nice variety of mufflers and pipes. The one time I did mine I used the original muffler and a straight piece of pipe out from the head into the muffler. A muffler clamp is good advice to keep from straining the cast iron around the exhaust port threads.
The Kohler carb is adjustable, no need to replace jet. If you have a Walbro without the top adjustment, I suggest finding a good Kohler carb. #26 for your 12 hp engine. #30's are for 14 and 16 hp engines. Hope this helps some. Great looking Cub you have! 
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DWayne
1973: 128, ag tires, 3pt. lift, spring assist, lights, 42" Deck
10" moldboard plow
2016 XT1 42" deck 18HP
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