Contrarian here... the problem here isn't the crack, it's the loss of structural integrity because the thin skin is bent. No matter what you do or how you brace it, the panel will continue to move and get weaker every time the door is operated. It's a highly stressed skin, and any bend will quickly become a crack.
If you can obtain a single panel, I suppose that's one option. The panel below the one with the crack may be compromised as well.
This will just get worse and worse and worse no matter what you try to do to stabilize it, especially if it's a single skin door. You can buy some time if you have to; when this happened to my door (cheapo uninsulated single skin door installed by the previous owner) I braced it with a steel framing stud for a while until I was able to install a new door.
Secondly, if that's not an insulated door, installing a double-skinned insulated door will make a frankly astonishing difference inside the garage. Working in the garage will be far more comfortable in any weather, and the stuff in the garage will last longer. It could be well worth considering an upgrade anyway.