Not sure about the 6K but the 4PLS and 4GRS are made for 6v and a 4-cell NiMh or an 2S LiPo is out of range. Also the shape of the battery is that well chosen that a 5 cell NiMh pack will not fit....

People are using LiFe packs (6.6v) or buy 4 NiZn batteries with a charger. Some do buy the manuals advised Futaba packs.

It seems it has already a pack?
https://www.manualslib.com/manual/10...k.html?page=22