Don't bother, most likely you have to modify a whole lot more that's way over your head and too much trouble.
I did it with 1.1 and 1.5 but as I said you have to patch a whole lot of stuff to get it working without the client disconnecting (crash) etc.
Attached is a screenshot of 1.5 and 1.1 CoD1 clients connecting to a 1.1 server if i recall. (could be 1.5, don't remember)