I'd replace. The tank is compromised and leaking. Heatpump, conventional or tankless regardless of your decision, replacement will be needed.

My recommendation would be to either use a larger sized self tapping screw, or go one size up with a regular screw. You could. Go the JB route, but if it's going to hold up a fan I would rather one of the other two options.

Or replace the box. if you're comfortable shutting off the breaker and switching everything over.