I’d have to say leave as is personally.
From what i’ve been looking at way too many of the sticks are going overboard on obnoxious effects.
But, im assuming the person requesting it gave you that piece with the button placements already? (if he didnt, move the piece so that ryus face isnt being covered, you could do that by flipping the piece entirely. Kens situated in a way that it would be on his chest, though i’d be sure to RE flip the signature and replace it in another spot as an ode to the original artist)
But, if you or the person insists on adding to it, find brushes that accent the scratched, almost scrapped in appeal of the lines between akuma/ryu/ken. You could do this by doing it on two layers one for the reds underneath, and then one for the blacks. You can use the pen tool to get strokes ya want if you dont have a tablet. (or by chance your hand with a mouse)
I’d really suggest moving the art though if you can so its not being destroyed by the buttons.
Also, if the sticks casing isnt white or has a different color bordering the pic, you could do kinda like i did with this p.o.s i was makin
So that it kind of blends in in a fun way. Kind of makes it look like the image is worn away on the edges.
Thats all i got off the top of my head anyway.