Am I Being Unreasonable? Original Owner Wants Car Back

So, I was lucky enough to find and buy back my Dads GSS Demon in 2018. The owner told be the back story and I completed it by saying who he bought it from and it was my dad's car. He blew off the other 4 potential buyers saying I get first dibs. I paid asking and didn't question it. I found where the #'s block, I offered a different block, machining money, etc. It is in a running car. He is holding it hostage, $5k for a bare block. Some are just assholes. For me, sorry, sentiment means too much. I would make a very reasonable deal and look for your dad's actual car. There is nothing like the actual car. I wish I knew the separation in numbers. I would find a way to get it to him without losing too much. Maybe less drivetrain? Just me.