Someone sold me an iPhone with find my phone on,

I bought an iPhone 5s for parts with a cracked screen and find my phone on, witch means it cannot be set up as a new phone unless you turn find my phone off using your iCloud password through iCloud.com. I contacted the seller and asked if you could turn find my phone off through his iCloud but he said he found it on the side of the road with a cracked screen and he had know idea who's it was or what the iCloud password for it was. When I heard that I knew there was little chance that I could make this a working phone again so I was wondering if I could replace the motherboard/data storage/logicboard to make it a brand new phone with no data remaining from the old phone?