Good Hardware for sleeper pc

Hello all, I have recently been looking into turning my xbox 360 slim into a "sleeper" pc. I don't know what kind of hardware would be good for a mid range gaming pc that would fit in the xbox 360 slim chassis, that also isn't ridiculously expensive, (Preferably under $650) and doesn't really need a gpu (Ill be playing games like minecraft and some basic steam games, maybe something like fortnite.) Does anyone have some recommendations?

P.S I already have a basic gaming pc and want to do this as an extra project so please no "just buy the parts for a regular pc and build it"