Charlie is right, from memory, the original body is the Protech body, which makes the buggy look more like a real buggy.
Just scour ebay, you will be able to find an original chassis (unless you want a championship edition, buy both

)
Here, I'll start you off:
http://cgi.ebay.com/Vintage-gold-tub-chassis-RC10-offroad-buggy_W0QQitemZ280417365978QQcmdZViewItemQQptZRadio_Control_Vehicles?hash=item414a2d6fda
Team Associated still sells some of legacy parts, but most are NOT for the original RC10 (although it will fit), but for the Championship Edition.
I have a RC10CE, the difference between the RC10CE and the original one, in terms of geometry, were the arms and the tire/rim size.
The original RC10 used a 1.7"/1.5" f/r rims, the CE used a 2.2"/2.0" f/r rims.
There's a faq on AE's site that talks about how to use the modern 2.2"/2.2" scheme.