You may have to look at Kteller and Thermal R&D
http://store.kteller.com/kteller/kteller_complete_exhaust_systems/g-62619.aspx
http://www.thermalrd.com/
I like the one this guy has: http://www.g1teg.org/viewtopic.php?t=516
They may have something up for sale on that forum, don't think there is too many G1 owners on this forum.