The wording on Magic Shop says "... an attack as a Reversal..." now I read this and take it literal but I'm wondering it either it is literal by using just an attack for the reversal step or if it has to be a reversal keyword.

Thank you for support.

The attack must already be a Reversal. Magic Shop says it allows the player to "play an attack as a reversal as if the attack had been blocked", so all you're doing is changing the condition for playing said attack as a reversal, removing the requirement to block the attack first.

If Magic Shop stated "any attack as a reversal", I could play an attack that does not have the Reversal keyword, right?

Yes, that would be correct.