If the engine has just seized due to lack of oil or a spun engine rod bearing or bad main engine bearings then yes the seized engine can be fixed if you have the know how or the money to pay someone to fix the seized engine.
I've repaired a seized truck engine that seized up due to a spun rod bearing and I just replaced all the engine bearings including the main bearings and rod bearings.
I also had to have the crankshaft turned because the crankshaft got messed up due to the spun rod bearing.
But the engine is now running again and has went over 20,000 miles without a problem.
If the engine has just sat and seized due to rust in the cylinders or the engine has other major damage then the engine may not be able to be fixed.
And also you have to see if the cost to fix the engine that is seized is worth it because if you have to pay someone to fix the engine then you likely could just buy a rebuilt motor or even another used car.
But having a rebuilt motor would mean the engine would last a lot longer than the used car you would buy.