Considering it melted down from lack of/low oil the bearings and seals are gone too, If the oil did not have a metallic look to it the lower may be salvageable .
To rebuild it you need the tools, the knowlege and manual to assemble it correctly. By the time your done a SEI upper makes a lot more economic sense.
The other thing is the teeth when softened and push downward can cut into the case rendering it useles