Remove the lower unit to determine if it's the power head that's locked up or the lower unit that's locked up. If you remove the lower unit, and you can turn the flywheel, then something is wrong with the lower unit. You can find a used lower unit fairly easily and not too costly.
If the flywheel still doesn't turn after removing the lower unit, then you have some damage in the power head itself. You would have to inspect/disassemble the power head to determine the cause and possible cost for repair.
