DT400B rebuild is underway, the exhaust pipe is dirty inside and not sure if there should have been some sort of baffle inside the silencer. Anyone refurbished one of these? 

Welcome from NZ downunder if you new or returning.
Likely the main pipe would benefit from a burn out to get it good & clean.
The tail silencer should have a spark arresting piece that's spot welded in if you look in there with the end removed.
That need burning out too as is often the main cause of lacking power & revs.
Here's an example i save from the net once.
photos.app.goo.gl/pdBfsFv268j8oDNT8
I just put the muffler in the log fire.
photos.app.goo.gl/BPj4ChxWtqNDmdURA
Sometimes i've put the complete main exhaust in an out door bonfire i had going down the farm.
But at work back in the day we'd burn then out with a propane torch & compressed air.
Had a tall chimney to take it over buildings but must've dumped it on someone.
Cant imagine that going on these days in the middle of town.
Here's a real bad case of the header of a little 100 twin i had.
photos.app.goo.gl/Wec5rDRziuSKL9R46

 

Thanks mate. Yes i can just barely see the spark arrestor, all gunked up. I will try a burn out (when nobody is looking).

Just re-thinking, where that tail muffler engages in the main chamber carbon grows there too where the tail muffler pipe ends, especially if the muffler itself is getting bad. So eyeball down in there & best find a steel pipe that size & push it through to be sure to be sure so to speak. Then try to shake the shrapnel out the front end. That's if ya not going to burn out the main chamber. All good.