well, i'd say get a yz 250 pipe (any kind) and hack and weld it to fit, but make shure u don't screw how it flows up, or u could get a out of frame drag pipe and custom mount it
send a message to yz otis....im sure hell help ya out
none of em is gonna just fit in there your gonna have to cut or notch some pieces to fit it around the frame. i have a fmf gold series head pipe that im using on my swap it would have took a little bit of cutting and adding in some pieces to make it fit so i just modified my frame and now my pipe will slide in and out with no problem