HobbyCity sell a small 2-cell soft pack lipo that will fit. They are about 2200mA and cheap.
You can also use the small hard case lipo that are designed for the 1/18 scale TCs and buggies. these are 1800mA.
Or
You can just squeeze a 2S pack of the A123 26650 cells in the 12r5.1.
When two are wired in series you end up with a 2300mA 6.6V pack.
You need to run the shock flat and change the rear shock ball to one where a bolt runs up through the top pod plate and through the ball. Xray T2 lower shock ball and ball cup works.
This is what i am running in mine and they are great. You can fast charge the cells. With a 21.5 run time will be ages (my 10.5 is well over 10 minutes), it will have a serious amount of punch and you do not need a receiver pack or booster.
Enjoy.