You're absolutely right, I've decided to try to implement existing game with already established rules for my first game entry.
Also the fact that 7x7 grid + 1 pixel grid fit perfectly in 64x64px requirements made me think it was a sign! :D
As for AI, I have to admit it's basically just random right now. I've implemented the underlining code, but ran out of time to implement the more advanced scoring like chasing the king when considering next best orc move. Once the rating period is over (I have to say, I'm not sure with game jam customs, assumed I should not update the game until everyone has seen the original entry), I really want to at least implement the system I had in mind to see if it makes the game more challenging (some Android versions I downloaded always play same moves, so I want my version to be better than that), but if that makes it more fun, I might expand the game out of jam rules.
Thank you for your feedback :)