Is there any way to raise the ceiling of a pre-built map? (say death island or something) What I mean by this is to raise the highest point where flying vehicles can go without being pushed down.

On maps that have a barrier that just pushes your vehicle down smoothly, such as Death Island, raising the ceiling is simple. This barrier is just a value at the top of the map's BSP tag that defines how far up from z = 0 it is. To change this, open the ".scenario_structure_bsp" tag and at the top should be a field called "vehicle ceiling". If the map does have this barrier, that field should be set to some number other than 0. In Death Island, it's set to 36.75. You can either raise the number to raise the barrier or just set it to "0" to eliminate the barrier completely.

However, all maps have a maximum height that's defined by the height of the BSP's skybox when it was modeled in Max. When flying vehicles reach the top of the skybox, they just smash into it, because it's just as solid as the ground. This can't be changed without remodeling the map. But, if maps have a vehicle ceiling and you remove it, you should get a good deal of extra height anyways.