If you can handle a soldering Iron, then all you need to do is find a battery that has same dimensions as what you have now (or what you know will fit your gun). Then all you will need to do is change the connector to a Deans.
You can get Deans connectors from Hobby shops, they are commonly used in Model airplanes. If you are in a jam, PM me and I will send you one.
For a fantastic intelligent charger, get one of these:
It runs on 12 V power supply, (or connect it to your car battery)
or you can get a PS here:
This will charge, Nicad, NiMh, Lipo and LiFe and Pb, at whatever rate you want ti charged. It also displays what was put into the battery so that you can see how much charge it took. They also have the ability to discharge(cycle) your battery to condition them back to maximum efficiency.
I have 3 of them. I use them for my model airplanes but they are perfect for my AEG's as well. This is a fast and easy way to do it.