its not worth the money to strip down the engine to see whats wrong with it, it could either be a bent conrod or valve.
If you get a workshop manual for your car it should have the procedure on how to change a headgasket, but i wouldnt attempt it without some godd mechanical knowledge.
the reason is costs so much is that alot of time is required to strip down the engine to get the head off, once the head is off is also needs to be machiened at a specialst workshop. getting a workshop the change the headgasket usually costs upwards of $600+
there could be other problems in you engine that you dont know about since its hydro locked, maybe bent valves or bent conrod.
You are really better off getting a low kms 2nd hand engine with warranty

