Are you trying to use it as a saw or are you going to convert to an airplane motor? The "120" designation refers to the model number of the Sachs saw. The 120 and the 121 saws are listed as being 61cc and this is in the 3.7 ci range. The bolt pattern for the two muffler bolts (centerline to centerline) is 1 and 9/16". If you are looking for something simple to strap on it for aircraft use you will find that Bennet (B&B) has a two pipe muffler (kinda loud) for somewhere in the thirty dollar neighborhood. If you need a saw muffler, pretty much anything Husky or Stihl has to offer for that size range will work. Sachs parts are getting a little on the scarce side of things. As far as the primer bulb, contact Walbro and they will be able to fix you up. (If you are going the airplane route, replace the primer bulb/plate assembly with a standard plate and do away with the bulb.) You'll choke the motor by putting your thumb over the carb intake or use the manual choke anyway. Hope this helps. Our resident saw motor guy will no doubt see this thread and will give you more info than any 10 of us could.
WT
I just re-read your request and wanted to tell you there is a difference between a 120 and a 120 Super. I also found some places to look or call for parts and info. Check on line for "Chain Saw World of Bowling Green." Also you can call these two numbers for parts: (704) 289-3586 or (715) 528-4439