We won't release a replacement like that since we want the weapon to be used in the context it was designed for, however it is possible to do and it's no secret how to do it if you know a little bit about how to mod. The model just needs to be renamed to plasmagun.md3, and the appropriate graphic and shader files exported into their own pk3, and of course the sounds would need to be repathed as well if you wanted the sound effects. Keep in mind it won't colorize though, so you're on your own there and you'd probably have to colorize the plasma tube graphics (normally the gen code does this, but Q3 won't). Bear in mind you cannot distribute our material per the EULA included, but what you do privately is your business. Just so long as you don't break your Gen install or infringe on our copyrights or intellectual property rights, experiment at will.
I exported Generation's player skins to my baseq3 directory.