Seat post clamp bracket failure - how to repair?
It is aluminium, it is a material that do not like welding, even less drilling through it.
In my opinion, you are left with one reasonable option only, as mentioned by @mattnz, remove the other half of the bolt clamp and then use a new clamp.
Please use the longest seat post you can find, to avoid stressing the top part of the seat tube. Given the failure you had and the laid back posture on a cruiser bike, it may be helpful to place the new clamp with the bolt in the front, not on the back.
You have also a crazy option
: banana seat, as explained in this video from Youtube.com