The $1800(?) is, exactly like you said, a sunk cost. Don't fall prey to the sunk-cost fallacy. It would be a mistake to try to 'save' that money by putting in more work.
However, if the vehicle has value to you/your family as a car, then it might be worth fixing.
1) Get a leak-down tester as has been suggested. This will tell you exactly what's wrong and thus what your options are. If you find a bad valve you will need to pony up for head work at $100-200ish, if you find a band ring/piston you might have to buy a new engine or rebuild, or you might confirm the bad HG theory.
2) If you don't part it out or sell it... Work on the engine in the car, or yank it and do external work where you can perform multiple possibly over-due items like oil seals, water pump, etc. depending on how froggy you want to get.