First off, that #14 wire should NOT be on a 20A breaker. It needs a 15A breaker, so you will definitely need to replace the breaker.
I would just get two single pole breakers and have one as a spare. Or you can get a filler to close up the hole where half the two-pole breaker was.
These fillers, same as breakers, are normally panel specific.
You move the white off the old two-pole breaker and land it on the neutral bar.