2500 is a lot of money to spend for a boat anchor.....
if the headers are toast, then the heads are toast including those nice valves.. that does not explain the motor not wanting to turn over..
I can almost garantee that the block is warped enough that the crank has frozen, that would require rebore of the mains and turning the crank by at least .030... by that time you have lost reliability from the pressure that you are going to apply....
The motor is worth two things, the value of the used accessories attached and the value of the scrap metal....
do yourself a favor, don't go there - he should be paying you to haul it away.
KC