You cut the tack welds ? Completely ?
How much of the insert protrudes would be the next thing .
If there is enough a pipe wrench to twist it or a good quality vice grips .
Once it turns you should be able to lubricate it and turn it , then work it out via wiggle turn and tap .
Failing that a self tapping bolt or weld a bolt too it .
Another alternative would be heli coil , time sert , oversize bolt with new thread tapped, or even metric size . Lastly weld a same size stud in the insert .