You could try and machine a 45 degree chamfer on the underside of a std crown race, so it sits on the integral fork race.
Issues with this would be-
1, the steerer tube may not be long enough to clamp the stem on properly because-
2, the forks would effectively be hanging out of the headtube by another 4mm or so.
The sealed external headset would be best though, I have one on my Alone which cost less than a tenner on ebay, pretty good too.
MM