Campagnolo on a Huffy? Awesome!

I was given a Huffy MTB (they are really rare over here in the UK) and although it was a lump of rust on the outside, it cleaned up to sound bright bare metal, so I sprayed it gloss black. It's so hideous it's brilliant - the seat stay joints and drop outs seem to be made by crushing the tubes! I'm planning to build it up into a "disposable" beater. Only problem is I had to saw off the black painted steel handlebar stem to remove it, it was really well seized into the steerer. None of the stems I can find are thin enough to fit the steerer, should have measured it first? Does anyone know what diameter stems these "bike shaped objects" use?