If it's a newer one that is already drilled and tapped for an optic go with an Allchin mount and a reflex sight that fits the mount variant you pick. I always have had good luck with Burris Fastfire 3s.
I have two Ultra Dot L/T reflex red dots I bought years ago and they are still chugging along. The best thing about these (aside from working forever) is the side battery tray that makes battery swaps painless. Changing the battery in an FF3 can be a challenge sometimes when the battery cover tries to cross thread itself. The biggest flaw with these sights.