How far you go with the rebuild will determine how time consuming it is, along with the size of your wallet.
I may not be as bad as it looks actually. You can buy a new SS bolt kit for the engine and get rid of the rusty nuts and bolts. You can remove the side covers and clean them off with a scotch brite or something similar.
First thing I would do is look in the spark plug holes to see if the cylinders are rusted. Get some diesel or Marvel Mystery oil and dump a couple ounces in there and let it soak for a couple days. Remove the caps on the head and examine the cams and dump some oil all over them.
Resist the urge to start ripping everything apart. Be patient lol. After the oil has soaked in the cylinders for a couple days, try to give the crank a turn, You will need to remove the side cover to do this, or you can try turning the back wheel in gear.
If the cylinders or cams are rusty, You might be better off with another bike.