Unless there is new growth emerging, those areas are probably dead. There's no easy choice - either accept the brown look, or cut out the dead and have half a tree. Covering with plastic is not a good idea as it could trap moisture and be detrimental. As for the other one, there is also no easy way to make it look natural. You can cut it back - 1/2 is pretty severe - usually not more than 1/3 should be removed. Once you cut it, it will look like it was hacked back. If you don't quite as much, you can try and reshape what is left to a cylindrical shape. Cutting it to the ground would not be advisable - it will take years to re-grow. You would be better off starting with a new plant.