I would look at the equivelent of a Speed 600 motor in a brushless.

Be advised that you will also need to change the ESC ($$$) and if you want the "45 minute" flights you WILL NEED to go with LIPO batteries and charger ($$$$$$).

You SHOULD be able to get away with a 2s2p or 2s3p (4000 - 6000 mah ). These should provide the flight times you want.

The motor and ESC will depend on whether you are going direct drive or geared. The geared will provide a lot more flexability on motor size and prop size/pitch.

Very rough guess on cost is going to be in the $350 - $500 range for the upgrade.

If you get MotoCalc or ElictriCalc you can play "what if" with various motors, gearbox, props to see what will work best for you. MotoCalc has a free D/L that is good for 30 days.

Hope this is some help.