The motor should be fine.

The batteries should be fine, might get a little corroded.

But the controller as well as the throttle mechanism will not like the rain at all. If you get the controller out of the rain and the throttle grip is cleaned and lubed you can try placing a bag over it and you should be OK. Maintain it, and spray it down with electric motor cleaner and re grease it nighlty to keep it water resistant.