Modify a STL file by removing some of the detail

I have an STL file of the batmobile , I would like to remove some of the detail in the model so can cut it with a CNC machine out of foam. I want to make the wheels smooth with a very small recess showing where the wheel well is, I want to remove the fans inside of the holes. I want to remove the tail lights, I want to make the roof vent smooth without a hole in it, I want to fill in the holes under the vents on the rear deck lid, I want the rear panel where the exhaust is coming out to be flat, there is a fake looking motor on the sides, I would like this part to be flat without any of this detail. I would like the vents on the sides to be very shallow without a hole. I have uploaded a screen shot, I can email you the STL file if you need to see this. It is more then 25 MB so it will not let me attach