I don't have experience myself of those sizes, but the dutch guy on BromptonTalk (sorry can't recall his name right this second) who sells and modifies them has fitted 17" moultons in the past - perhaps search the archives or knock out a quick question to see what experience people over there have.
Shame to hear about the TI brazing. Somehow I knew this wouldn't be a simple thing. Seems to be the reserve of the uber-rich, the military and F1 teams then haha.