Yep. a lipo and only one specific sized one will fit the 15 bodes. KWA's are built to run on 7.4v lipos. So just get that voltage. (if you can find one they are hard to find in that size)
These are your best bet.
If you decide to run that gun with an 11v
Here it is on Evike.
It is out of stock so another option would be