You can buy a retro fit and put it in your canopy but Gsellers was telling you that one is not a good one. Personally I don't know since I bought an entire fixture and do not have a hood.
I'm not sure if Jack was talking about a fixture or a retro kit but I would bet he could get either for you. There are different quality in ballasts,reflectors, bulbs pretty much everything. If you are going to put your own together you will want the best of each thing.