There are two different subjects you are touching:
- The air filter box
- The crank case
As I agree with you on the air filter box that water can enter through rain. I disagree with you on the crank case condensation. There is NO issue with any crank case condensation on motorcycles. The temperatures will literally boil off any condensation water if there was some and the bike is used.
The only time this could in a "very theoretical" way happen is if the bike has been sitting for a long time, not used, and in extreme changing temperatures outside, hot and cold, and wet. Or, if there is a damage on a hose, or the casing is cracked, and water enters the crank case, but not the way the engine was designed.
Regarding crank case condensation, you are looking for something fictive and that is
not a problem on Kymco 250 engines.