Frames of this vintage would be dirt cheap if you could find one at a thrift store
or garage sale. New equivalent frame from Nashbar might be $150-250. Doubt
repair cost would be worth the expense and would necessitate a repaint. Welding
is likely to distort the tubing too much, even brazing likely would: this is where
a moderately precise part of the bike fits, the headset. Replacing the lug and perhaps the head tube would be the best approach but not worth it. Find another bike. Steve