I've made
ceviche with tilapia once using a Rick Bayless recipe that originally called for mackerel.
Tilapia's neutral flavor, in my mind, makes it a less-than-optimal choice for ceviche, but it won't be bad by any means. Since you said that this is "leftover" and "okay", you certainly don't want to give it a light dress and serve preparation--you probably want this fish fully cooked which will require an acid bath for at least 4 hours and as much as overnight, depending on how big you chop it up. Don't marinate it forever (keep an eye on it). Too long in the acid and it'll dry out and become inedible.
Just before serving, add some tomato juice, diced tomato, red onion, and chiles.
Best,
Michael