Does an aircraft need special modification for polar routes?
There are several considerations for flying polar routes, but no special modifications are necessarily needed. Navigation is different, as Lat/Lon (specifically longitude) converge to become meaningless close to poles, so aircraft traditionally use a grid navigation system. Aircrew should also be aware of the difference between magnetic and true north, as the magnetic "North Pole" is different than the geographical North Pole.
You should also understand your aircraft's temperature boundaries. If you fly through the arctic in the depth of winter, you should ensure that all of your aircraft components are suitable for the temperature. Usually, this isn't an issue - and if it was, you can always descend a bit to find "warmer" air.
Otherwise, air provides lift in the polar regions just like in the equatorial regions, and Bernoulli keeps your aircraft afloat!