Get it on the ground- should be the primary concern. Land flaps 20. CRJ-200 WILL NOT FLY single engine, flaps 45. Quick math- double landing distance and consult Vref flaps 20 on the card you have for your weight. Run the card if you can- stop there.
Since when were you single engine? This is an engine fire, and until the Eng. Fire switchlight is pushed, you are most likely producing thrust. By the time the QRH is run, you might be on a 1 mile final.
So the approach should have been briefed before the LOC is intercepted. Are you going to rebrief, or, from your systems knowledge and Captain's authority, modify the approach to the current needs. Flaps 20 or Flaps 45?
Just want to be clear that an engine fire does NOT mean single engine thrust, and non-book (that pilot stuff) decisions will need to be made within the very short amount of time you have left before landing.