Why ignore the old bolts? If they are not compromised they are fine. Go with the first suggestion, drill a new hole next to the broken bolt, insert anchor and tighten. You can get the epoxy capsules that drop in the hole, when you drive the anchor in it breaks the capsule and locks the anchor in. I've done this many times on equipment with higher loading than that basketball goal with never a fail.

Its possible the 3 remaining anchors would hold... but put the fourth in anyway.

C