File the remaining threaded extension back as flat as possible
Get a male/female nipple ( I think puritan Americans call them joiners cause their tounges will fall out if they say nipple ) then either use the red loctite on the nipple or a good wack of thread seal tape ( string would be better )
Or
Use the red locktite on the plug and never remove it ( use a pump to change your oil )
or
Buy one of the oil drain extensions ( there are a lot of them ) and again fit it with red locktite and never remove it
No glue , putty or paste will repair that broken off part because the taper on the plug will break it off.
Because it gets too hot the aluminium solder will not work as it will get soft & fall off , even if you clean the surfaces enough for it to take
The final alternative is to get it welded & recut the oil hole.
However unless you have a mate who works for a bottle of burbon a new sump casting would be cheaper .