No way to tell until the engine is on the stand and opened up. If the cylinders are fine and not worn and crap, a simple hone and new rings would do the job. But God knows what broke in there and what things look like. You can use cheap Speed-pro pistons, they will do the job and aren't too expensive. You should give RPM racing in laval a call (450-978-6637) they know their Mustangs and could have used blocks for you? If not there's Zeke's, they awesome at engine building (http://www.zekesauto.com/en/index.html).