Daniel,
Here's a starting point for you....
from Macs Products:
I do make a header that will fit. The exhaust port is a little bit smaller (21.50mm
wide & 9.90mm high) but the screw holes are the same. Its the #2985, #2986 or
#2987 these are made for the YS.91 SR motor so they have a 7/8" tube just the
size for your motor
http://www.macspro.com/headers_side.asp