Open the servo's and put some (glueing) kit on the edges of the case and close the servo's. For me it is a standard procedure because the standard sealing is not protecting enough.
The receiver can be putted in a balloon with some cleaning putty:
If there is a lot of rain you can use the coolinghead of an airplaine engine:
Inside the insbox there is no airfilter, a foamfilter can hold water and block the air. Only be sure the dirt from the tires can not go into the box.
Then with some Kiss raintires, the car will look like this:
If the track is very wet you can lay a sheet of lexan under the motormount so the heat will stay in the motor. Also use a warm plug like a 6 or even a 5
For just a wet surface you can use an Aqua tire, this is a soft foamtire mostly with 15 to 20 shore. And if you can handle the power and fun: