I must be missing something in the question but if there is a female thread on the exhaust=screw on. Round type is fine. This image is on the forum here somewhere and, although a different mower it is similar and was the closest looking one I could find.
If the exhaust port has a thread in it, you can use the pot type as above, or the "low tone" long tubular type, as long as they don't hit anything, which they shouldn't in your case. The pot type is rather loud, though.