CG test--you basically have it right. Pick it up with your figertips at the recommended CG, and start moving around weight until it balances level. Usually the battery is a good thing to move in order to try and balance a plane.

There is no reason at all that the 70 FS will not mount on the alpha. However, you might have to get a new mount, or re-drill the one you have. There is a good chance that the bolt pattern isn't quite the same. I'd suggest just buying a new one. Every hole in the mout will make it weaker.

Also, know that the plane will not fly near as fast with the .70 FS. The .70 FS is near the bottom of the recommended engine for this sized plane... But it will fly it!