it's very doable. just go to ebay and get a bmx seat clamp in 28.5 diameter
https://www.ebay.com/sch/i.html?_fro...+28.6&_sacat=0
cut down your steer tube and get a low stack height head set if you have to. remove the star nut, of course
however, the simplest solution is to get 17 deg threadless stem as badger said. again, a low stack headset if you have to