You'll either need to replace the joists with larger ones, or you can do what's called sistering, where you add another joist at the location of the existing one. That way, your joists will be doubled up—or sistered together.
Talk to a contractor to be sure that your ceiling joists are sufficient to work as floor joists. The existing joists are most likely sized to be sufficient to hold...
If you have any power lines running to your house, prevent rats from travelling along them by installing a baffle. Also, clear out any piles of debris or clutter around...
