every state is different so not sure this is the right place to be asking for assistance. what works for us in nevada might not help you in texas.
if it was me, I'd spend some time with the dnr there in person and see what they can do for you. they should have the old owners' contact info since it had to have been registered at some point. I assume you have a bill of sale from the guy you swapped? that all helps your case. my experiences with dnrs in wisconsin/idaho/nevada have been much different and more willing to help than dmvs and with your case being not the norm, they might have a supervisor that will help you get it done. likely not going to be easy tho. I see several hours of your time involved in this.