Like Rambozo said, 304 and 316 are available. Locally they're mostly 316 for me. Either 316L or 308 filler would be fine. Whatever you have, assuming the exhaust was made of 304.

If you're welding steel to stainless, then 309 is technically correct. It has extra chrome & nickle to make up for the metallurgical soup you make joining mild steel to stainless (turning your soup into "not quite stainless but better than stainless dissolved in mild steel"). But in real life, 308 will work if you have some already; your soup will just not be quite a stainless.

Bungs down to 1/2" are common in the plumbing supply places, which is the only reason I asked. Smaller might be hard to find. Remember pipe sizes are nowhere near what you actually measure. (1/8" pipe threads are about 3/8" OD for example).