Something like that today would be a Civic or Corolla with a lot more bling and features, and my blind guess would make $1,400 the minimum possible repair-market price. Then, I was young and fulla beans, and at age 33 was able to carry the engine from my parking lot to my cellar storage bin - or about half a flight of stairs going up, and a whole flight of stairs going down.

The Civic 1200 was considered by some good-ol'-boy Dukes-of-Hazard-type mechanics to be in a muscle-car class by itself, but the engine was small, the vehicle had a simple design, and an engine-overhaul expense reflected all of those factors. When I once had this sort of thing done, I was recommended to overhaul the entire engine, which had ~160,000 miles op it, and the engine overhaul expense at that time was approximately $700 and change. Here's a decent description: Replace Head Gasket. Even if your decision ultimately favors outsourcing to your mechanic or shop, it's not a lot of trouble to familiarize yourself and give a quick glance to what has to be done to replace the head gasket. This would likely be a large belt-driven sander, which can support the size and weight of the relevant parts. It's not an easy DIY operation even for seasoned home-mechanics, unless they have easy access to a means of resurfacing the cylinder head, where it mates with the engine block. You seem to have addressed all but one: a blown head-gasket. Overheating can be a symptom of many things. Also, check your oil-dipstick, and see if you nave antifreeze with water in the crankcase. Do you see something that looks like oil, or an oil-emulsion with the coolant? I think this would look milky-white. Let the car cool off to room temperature, so the radiator is safe for cap removal.

Buick century oil in coolant