Traditional "forklift" brushed series DC motors all appear to be open: you can see the commutator and brushes. Some builders use a commercially available or home-built shroud to close that opening, and provide a connection for a blower, with a filter on the blower intake. An appropriately placed filter will prevent problems with water.Honestly no experience with Fork Lift motors. Could not tell you if they are sealed or not.
Search this forum for "blower" or check suppliers such as EV West for a motor cooling kit, and you can see examples.