I have a 1980's low-end Mondia without a derailleur hanger. I would like to know if there is a way to add a derailleur hanger so that I can use more modern derailleurs. I plan to change the gearing so I have to change from a short cage derailleur to a long cage one. Without a derailleur hanger the only derailleurs I can use are Shimano Tourney, which I would like to avoid. Can I weld a derailleur hanger to the frame, or the heat would weaken the steel?