Call Desert Aircraft - get their recommendation on a Pitts muffler -
they do have access to a nice one - I believe it is a Johnson - not certain/ As for vibration - your DA 50 viberates just as much as my ZDZ50 - this is not speculation - it is simply how these engines run - you can insure that all balance ignition carb adj is all correct but the POWER PULSES plus the inherant unbalance in all singles at about 3000 will induce vibes that can kill welds -if the muffler is not fully suported.
I have run the DA50-s - and they are excellent engines.
Your problem is not unusual nor peculiar to DA.
If you have a mounting boss added - be sure to mount the support to the engine-- NOT the firewall!