Depending on the O.D. of the actual headtube you may be able to bore out the I.D. to fit a 1-1/8 headset (so you end up with enough wall thickness). However the thru dia. on the headtube may be too small depending on the wall thickness of the headtube, so the fork tube wont slide thru. I have done it on a few frames, I cut it close and finish the cut with a headtube reamer that leaves a smooth 33.8mm dia for the headset. You could weld a sleeve over the existing headtube and then cut the ID if the OD is too small. Alternately you can cut off the fork tube and weld on a 1" tube and not mess with the frame at all.
