If I was a good Android programmer, I don't know if I would feel good about releasing a game clone. But if someone is paying me to simply code it, I'd just take it as an assignment and get it done for a decent amount of cash.
I think if you are upfront about it, it should be okay. You can tell the client that you already have most of it ready, but you can rebrand it for them for a certain price.
Oh I am not an android developer. But hypothetically speaking, I'd make sure the buyer knows that I still own the code and he is just paying me for a packaged app.