I looked into this and am in the process of it.
If you have the ATI stock, they make a housing that goes in the stock to hold a laser site. Laser sites I have seen at WalMArt for $20.
If you are original stock, do a search on E-Bay for tri weaver rail. There are several out there that attach to the barrel and have 3 weaver rails to attach items with.
As for a light, that gets expensive. To be a good one, you will spend up to $100. That is due to brightness, a switch that you can turn off and on light a laser site, and size. Once again, hit E-bay for ideas.
I believe that the ATI mount is for a smaller diamerter laser and will not fit that mount. I bought a pair of mini rails for 12 bucks and mounted a $15 laser off ebay. The laser from Wal-Mart is better then the one I have and will probably end up buying it as well. The Waql-Mart laser also has the pressure switch that you really need.