know this is a old post, but for people searching, i'll put in a reply.
toyed with this idea a bit, but ended up getting FMF 4.1.
As far as i can understand, pretty much every pipe from newer KTM's will fit, although stay under 2011 models i think. as a reference, i bought a pipe from a 2010 KTM 530 exc, and it will fit by slighty grinding the brackets to get the exhaust to seat fully to the midpipe.
Not much work at all, fitted in under 5 minutes. other than the need for a longer exhaust spring, everything is good
