Nice idea, ebay has them even cheaper
They are larger than AA. 18650 is 18mm diameter, 65mm long. AA is 14.5mm diameter, 50.5mm long (fun fact: been around since 1907)
Looking at pics of the redcat:
http://www.hobby-estore.com/Redcat-R...uck-yellow.htm
It looks like there IS room for longer cells, if you're willing to play surgeon a little
If the AA holders are removable, less surgery than I'd initially guess.
But based on this review:
https://www.youtube.com/watch?v=O2mSkqCkvTA
The motor was getting hot with regular nimh, and people reported servo issues.
It may be optimal if doing so to be prepared to replace the electronics/motor/pinion/servo to handle higher voltages (unless your mod already included that?) since the nominal voltage is jumping from 4.8 to 7.4.