I just got engaged but I'm going to keep texting my ex.
Are you texting him in a friendly capacity or as a romantic partner?
Well, he's a great kisser if that says anything.
If you were just being friendly because you still enjoy your ex's company at times, that would be fine but it sounds like you're doing it for the sake of physical intimacy which is not okay. You've supposedly moved on otherwise, you wouldn't have gotten engaged, so you shouldn't be continuing that relationship in any romantic sense.
I just don't want to call it quits with my ex.
You can still be friends with your ex, but you should be ready to move on romantically.  If you are still trying to get intimate with your ex, then most people would say that you are not yet ready to take on the commitment of marriage.