Vape exploded near iPhone 6s plus

I was given a iCloud locked iPhone 6s plus for free. however, a Ecig exploded near the phone and wiped out the camera. I had a spare iPhone 6s plus same model another friend had given me that wasnt iCloud locked but the framing of the phone wasn't good, so I swapped the motherboard and the home button into the one where the ecig exploded near and was iCloud locked. atleast 6 times a day the phone randomly restarts. The back of the phone does get really warm quick so basically I've not been using the phone. I replaced the screen and I ordered a new housing with a bunch of preinstalled parts and a camera I'm wondering if this would fix It or do I need to replace the battery as well. I have no idea what would cause my phone to heat up or randomly restart like that? Any ideas? Also my battery Health capacity is at 83%. Could the ecig exploding near the phone mess up the battery? The ecig was in the same pocket as the phone when it exploded. It left black marks on the phone and messed up camera