How to scuff up epoxy primer in order to spray the top coat?

If your goal is to have the bottom look as good as the top, you need to apply high build primer and sand it smooth. since 'life got in the way', you do need to scuff the epoxy primer for mechanical adhesion. I have been sanding epoxy primers for many years without any problems.
If perfection is your goal, I would lightly dry sand the epoxy with 320. I am talking about the kind of sandpaper specifically made to block sand primer with. Then apply your favorite primer and sand it until it is ready for paint. I would use Evercoat G2 polyester 2K primer and sand with 320 dry followed by 320 wet and 400 wet. I'd go to 600 wet if I were painting metallic.
If you just want it to look good, I would lightly sand the epoxy with 320 dry followed by 320 wet and 400 wet (then 600 wet for metallics). Then, with proper mechanical adhesion, you can spray your paint.
PERSONALLY, I would go the primer route. It isn't that much harder, it will look better and you will not regret it.