well you can buy a new servo saver just make sure its splines match the servos.

As for batteries the Ni-Cd's are probably old as hell, and are generally crap even 16 years ago when I was really into RC's Ni-Cd's were rare due to how bad they were all around.

From what I see the RC came out late 90's early 2000's... so the electronics might not like LiPo's very much. NiMh's for RC's still seem to be sold. Basically mAh is run time, and cells are voltage. It all depends on your charger what you can get.
https://www.amainhobbies.com/ecopowe...-5023/p1532757 probably would work all though I think its a tad expensive might need a different connector as not sure if anything was done to the RC prior.