I've been through this several times in Virginia and North Carolina.
IF you can see the old registration numbers, or the number on the sticker, that makes your life a LOT easier.
Call the state, explain that you are trying to gain title and give them the numbers. Depending on the state, they may give you the HIN (if there is one) and also give you the process to get title. Typically, it's contact the last known owner, and if he has no interest, you get it.
I did a 16' Spectrum just this week. A renter abandoned it in VA a few years abo when they moved away. Property owner had tried to get title, but to no avail. I was able to find a shadow in the gelcoat of what the old registration numbers were, and they were N.C. numbers. I called NC wildlife, and yes, it was a NC boat.
From there, it was easy. They gave me PO's contact info and I had to attempt to contact him. Sent a certified letter to his home, and also found him on Whitepages. He had no interest in the boat, so I sent everything in, including his new contact info, and 3 days later, I had my temporary registration to use until the new one comes in with the title.
I have a 12' jon boat that has never been registered. Unfortunately, the previous owner refuses to sign anything. He refuses to answer my calls after getting paid. Because of that, I have to possess the boat for a year before I can apply for title. Annoying, but that's the law in VA. My time will be up next month, and then I'll get it done. Should be a great pond-hopper. I can use it now, but with oars only. Not into that!
