Dan's sells a number of European bottom brackets designed for BMX cranks. The Demolition BB is only available in Mid size, but BBs to fit 19mm spindles are a dime a dozen. I'd recommend either the Profile or Redline bottom brackets. Both are available to fit Euro style crank hangers.

One word of warning: I don't know that MTBs and BMX bikes have the same width bottom bracket shells. If they do, you should be golden. If the MTB shell is narrower, you can probably get by with extra spacers. If it's wider, the spindle may not be long enough to properly engage the spindle splines.