Weld, After careful consideration and sourcing the materials, I think you MAY be on to something....
I can get 1" OD mandrel bends easily (and fairly inexpensively)......
slicing it lengthwise with a hacksaw and inserting a shaped piece of sheet to spread the gap will expand it to an almost perfect 26mm size and create the reinforcing rib...(two birds with one stone)... I am considering brazing the bits together rather than welding as the seam flow will smooth the internal surface as well.....The flanges will be 1/8" sheet with 3/4" OD tubes 7/8" long (1/4" bore) to replicate the casting at the head end....With similar fabrication at the carb end to emulate its looks......
It dawned on me I can counteract the slight heat transfer by enrichening the jetting slightly....and I believe a 10mm high insulator is available for the manifold to head joint....
Opinions of this plan?......