I got an old TOMOS folder for $20 and I wanted to strip it down and make it a fixie. it has this weird press on BB set up with cottered cranks (not a fat one like on a BMX but small like on a road bike only not threaded). I would really like to put a new BB in this, but I'm not sure where I can find a new BB or an adapter. I know they make adapters for BMX bikes, but the ones I have seen are WAY to fat (diameter).
The same happened to me with my old Victoria folder. The BB was of the Thomson type (preseed races into the shell and adjusting cones) that restricted me to use cottered cranks. As you say, the existing BMX adapters didn't work because the diameter of this BB was way too smaller.
So I took the races/cups off the shell and went to a lathe-shop. They built a pair of pressed on adapters with the outer diameter of the races and the inner threading of an Italian BB (the width of the shell is 69.8 mm so I chose this pattern). Now the old thing is working on a Campagnolo BB flawlessly.