I believe the KTM fourstrokes from 2013 to 2016 will fit, 250, 350, 450 and 501 FE's use the same type exh as the EXC or XC-W Katooms. If you can get an exhaust off of a KTM SXF model that would be even bettter.
Its possible the silencers from KTM's as far back as 2009 may work. I'm not sure about the mounting brackets on these older KTM's if they will fit