Go for it if you want, but I would strongly suggest getting the transmission rebuilt with a fresh bellhousing.
Even if you weld it, the sleeve may have the potential to break off again around the weld, let along having it perfectly aligned.
Contact one of the Big 3 for transmission rebuilds and ask them how much it would cost you.
The part number is MD727855 for a new bellhousing, and costs ~$400 new ($575 is list price).
Try welding it up if you want, but you are going to need a damn good clean surface and use alot of acetone to get it to actually make a clean weld. Either way, you will need the entire transmission disassembled and cleaned before attempting to weld it.
If you just weld it on, you will torch your input shaft seal, and more than likely your intermediate shaft oiling guide. Let alone any other damage.
Just send it out for rebuild/repair and get a new or used replacement bellhousing.
1999 Mitsubishi Eclipse GSX