If the older game did not employ any kind of encryption of the data stream, isn't it open season to reverse engineer? So many of these are shut down by trademark licensing and artwork copyrights?
Some games have thousands of items/buffs/abilities. If the data for these is not included in the client (it sometimes is, usually for item descriptions) then it's basically impossible. The same applies to the damage algorithm. You cannot replicate the original game without the server code. You're basically building your own custom game that merely happens to share the same UI if you proceed to reverse engineer the server.
