Try Ebay for a used head.
You might be able to get a small engine or motorcycle shop to rebuild the head. If not seriously broken parts on the head, valves and valve guides(bushings) can be replaced.
It could also be the valves are out of adjustment which will make it hard to crank. If it is hydraulic lifters, then the engine needs to be opened up to replace them.
I also suggest a voltage drop test from the battery to the solenoid to the starter with a multi meter. Search youtube for a video for the principle.