pizman,
If you contact JTEC and tell them what engine you are planning to put in and which plane they will suggest a muffler for that application.

There is quite a bit of difference in power between the Zenoah G26 and the DLE 30. The 30 is stronger. There is also quite a bit of difference in weight. The G26 with elec. ignition weighs approx. 53 ounces and the DLE 30 weighs 39 ounces. On the Stearman the added weight of the Zenoah is a plus. If you use the DLE 30 you'll likely need about 12-14 ounces of lead in the nose to achieve the recommended CG or you could use a heavier ignition battery to save some lead. I hate adding lead, but the plane has to balance and sometimes you can't get around it completely.
As far as vibration neither engine is any better or worse than any other single cylinder gasser. Just make sure your props are balanced or they will vibrate every glue joint/nut/screw in the plane loose. Don't think the standoffs make it any worse. I'm running the stand offs that were included with the DLE with no problems.
As for the stack?? I dunno. I have heard both ways. Some say yes run it and others say no not necessary. Personally I don't use it and never had an issue.
PS: Pay no attention to manufacturer's HP ratings. I think they use the SWAG method (scientific wild ***** guess).
Hope this helps.
Bill