it looks like there are 3 bolt holes for that mount, and like you said there's still threads in the broken part. it would probably hold with no problems. theres two more bolts to share the stress, and use threadlocker you should be ok.
if you wanted to make it look nice put the mount in so the bolt is in and braze the broken piece back on, you wouldn't ever be able to remove it, but it should never come off either.