A worn brake is fine just tightening the screws a bit and re-adjust it.
If you feel the need just get another stock disk. I have tried all the others 4 different ones in all and went back to the stock.
If you replace it I would advise doing the brake mod. It will increase the performance and make the pad last longer. All it costs is a ballpoint pen. Here is a link
The Carbon disc brake by Traxxas is the best for less. It is cost effective and will definately make a difference in braking power. This disc also seems to wear a lot more evenly than stock.
The brake mod that Eddy has posted works VERY well. I think most who have read that mod have done it and liked it. I know I have.
The carbon brake will last longer. I found that when it got wet it drastically faded and I had a hard time stopping. On the other hand the stock brake grabbed a bit when wet.
Either one is a good choice and the price is only a buck or two different.
IMO Stay away from the aluminum disks. They use a pad that glues on to the caliper and they love to come off and ball up, forget running them in wet conditions. Changing the pads is not a field fix.
Here's what I did with mine.
I sanded down the disk a bit to get all of the shiny surfaces off.
I then took the edge of a file to the pads and made lines about 2mm apart at a 45 degree angle from horizontal. This gave the pads some rough edges. This bit of work took my Maxx from a stopping distance of about 30 feet to about 10 feet.