Probably at first a wider original fork. currently the original needs a 74mm hub (vs. 100mm "usual" size) - when adding a disc brake it would to be even slimmer. Not sure if i.e. Dahon or Tern do have 74mm disc hubs. Other than that I would not be sure if the current original Brompton forks are suited well to fit the forces from disc brakes (that are different from those of rim brakes).