hi there,
the original genx10 had a hole in the chassis for a removable tray that could accommodate cells and lipos. The new one is lipo specific with a solid chassis. Dont think you can get the original new any more but if you can pick one up second hand they are cracking cars. I use one and I have had some good results with it.
I have been using a 12th scale servo, which is what most people seem to be using.
You can use a full size servo but it will probably limit how far forward you can mount the battery.
have a look at my GT blog
http://rcgtpancars.blogspot.com/