well the 420lf is only a 3200kv motor and it works off a 25amp esc so you don't need that much battery or c rating to run it. I'm thinking the battery is just old and isn't putting out the rated voltage anymore.

You can upgrade but don't overdo it either, a nice 15c or 18c 1300 to 1800mah battery will be fine with that motor esc combo. you don't want to fry the motor either.

if you do want more power and flight times you may want to upgrade your motor/esc to the 430X/35G. at that point you'll need the higher C rating and larger battery.