What is the correct cord and plug for my forklift charger?
Sure, there is a mating plug: you need a L16-20P.
The charger may not have come with a plug because it is built for different voltages/frequencies, meaning it is sold in other parts of the world that have different standards for their plugs and receptacles.
NEMA sets the standards for plugs in the US, which is where the L16-20p standard comes from. Other countries use different standards.
I missed the cord side of the question, if you use a hard service cord like SOOW or SJOOW a #12 or 12-4 SOOW/SJOOW would be what I would recommend.  This is because of the nameplate rating of  13.1 amps @480v and the point that chargers are considered a continuous load.