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You can get a used Weber 40 DCOE on EBay for a couple hundred. I wonder, though, if those manifolds are a retrofit from something else. You might be able to measure the intake side of the Weber and the 450 intake and find a match.
Not a retrofit from something else. Brian Littler produced the manifolds and also manufactured a complete CB450 rolling chassis called a "Littler Honda". My recollections from 50 or so years ago are, it was a pretty nice bike with a nickle plated frame and equipped with top shelf bits like Ceriani suspension. The magazine article the photos are from, described the whole lashup in some detail. I believe that Brian lived in South Aftrica. Awhile back, I'd heard Brian (or someone) was going to bring the manifolds back into production, but never saw anything.
I was thinking something similar, the manifold doesnt NEED an S bend in my opinion, depending on the angle they could be straight pipes welded to a mounting plate at an angle. I've read up on custom plenum designs in the past, supposed to be smooth whenever possible but it might be a low cost starting option.
Four waterjet cut flanges and three pieces of tube on each side angle cut and all tig-welded together. I would think that will work just fine. Maybe not totally as good as a smooth "S", but I'm not sure it would be enough to notice.
I might mock it up in CAD and send it out for quote unless someone here wants to make a few bucks. Of course I'd much rather go with the original Littler parts if anyone has something in their garage..
Brian was living in Richmond, VA about 10 years ago. I don't know if he is still there. His email was email@example.com Good luck with your search, I devoted over 10 years to Honda building DOHC 450s and I've never seen on of his manifolds.
One of the guys at "ultra wheels" is into some pretty neat 3D printing in various metals. I cant recall his name, but talked to him a few months back about an obsolete wheel. He was doing 1 off and race fab pieces as a side gig. Might be an avenue to look at if you can find a part or a print. I know VW guys horde the old Webers so possibly look on "thesamba.com".