You can measure old battery and use a a lipo or lion with a ubec set to 6 volts.
https://www.hobbyzone.com/spektrum/all/spmb2200lfrx.html?utm_source=Google&utm_campaign=Google+Shopping&msclkid=6905edf89c0019a6f7348ea10286bca4...
higher mah dont give higher voltage.(only cell # does) Id go lipo with a ubec to limit voltage . they can be programed or use jumpers for voltage.
Id suggest do some reading on how batteries work