afvman:

Since the label would probably carry a WARNING rather than a DANGER or a CAUTION, I do not perceive the need for such a label. (See OSHA 1910. 145 - Specifications for Accident Prevention Signs and Tags.)

If the product of combustion engine hazard is properly covered in the engine/forklift manuals, the product literature, the fuel material safety data sheet, and in the operator training program; this should be sufficient to address the potential problem.