The Evolution 10cc gas engine uses an amazingly common muffler size. So you can pretty much get most everything to fit OK for a muffler.
From the Mac's Muffler website a list of mufflers that would fit OK. Thus choosing a Pitts muffler should not be a problem. I would likely go with something that would with the OS .46 through .55AX engines though.
Of course the Evolution Pitts muffler would work too http://www.horizonhobby.com/products...GX#t4-OPTIONAL

OS.40-.46 FX,SF, .46-.55 AX, .50 SX
-( also see the new .55 AX muffler #6720 )-
Evolution .40NT & .46NT-NX & Trainer A
ASP .46, GMS 2000 .40-.47
Irvine .40 & .46 (bolt through)
Magnum XLS.46
MECOA .40-.46, Royal .46
MDS .48 FS Pro (bolt through)
Rossi .45 (new)
Thunder Tiger .40 & .46 Pro
(center to center = 37.08mm/1.460")

or even these too:

O.S. .46-.55 AX, .50 SX (bolt through)
-(this muffler is a better volume match for the
.55AX, .50SX and the .46AX than the #6590)-
ASP .52 Evolution .52-.60NX
Magnum .52 XLS
Rossi .53 (new)
Y.S. .56 SR (new)
(center to center = 37.08mm/1.460")