Get Solid Fab to make a custom downpipe. he will need the manifold to work on so you'll need a JDM prelude and that manifold near him for about 1-2 days. Luckily I live near him
I had a reply from Alan at SolidFabs for the exact same question for when I was selling just a Mugen manifold. I wanted to know how much it would be to make a custom stainless downpipe so I could let a potential buyer know. Typically though the reply has been bumped out of my inbox on PUK, but from memory it was £200 and he'd need the manifold and car as Nitin said.
Alan (Solid Fab) did a group buy for OEM downpipe replacement and that was around £180. Not sure about the price now but something similar as its only half the actual manifold? Best to contact him and ask him for a quote.