OT: I'm working on an old water cooled car and have a question., Gasp. Options

[Krieger]

I'm doing some work on a friend's 1964 MGB. It has sat for the last 5 years. Before that in 2021 after sitting for 17 years a shop got it running and driving. I don't think she drove it more than a few hundred miles lot of reasons. So I am going through each system. I'm working on the cooling system now. I drained the coolant from the radiator. I tried to drain the coolant from the engine block. It had a brass valve that was stuck so I removed it. Nothing came out! Its totally plugged. The threaded hole (about 1/2") in the case goes back an inch and makes a 90° turn. I've tried working a bent wire inside. I can get up there about an inch. Still nothing came out. With the thermostat removed and using compressed air, I can force air into the coolant drain hole and there is some bubbling happening where the thermostat was. Not enough bubbling to justify the 50 psi I put in. How can I get this flowing again? What have you guys had luck with? I was looking at Thermocure by Evapo Rust.