The iPhone is capable of charging up to 1.67A on a standard 5V wall charger, so if you have an iPad charger (12W) you can use that.
The issue with the charging is due to the liquid damage probably near the charging port connector or a corroded charging port assembly.
Worst case scenario is that there is liquid damage on the board and it's fairly common for the charging ic (tristar) chip on the logic board needing replacement after liquid damage.
If that does need to be replaced it can only be done by someone that does micro-soldering / board level repairs on iPhones.
There could be more liquid under the metal shields on the logic board and may be slowly damaging the phone over time. You may want to open up the phone and see if the phone is still wet, has signs of corrosion (white marks) on the connectors of the logic board.