Cut off the lower part of the remaining structure. Weld on the new section. Weld on the outer panel. During the welding, keep test fitting the door in the car to ensure good panel fit. Complete floors are available for most types.
Problem is the edge on which the floor lies.
When the floor is gone, this part of the sill will have gone as well. Original driveshafts will keep good when treated good. A very large press is needed to get the silentblocks out of the front axle. Be carefull because the cradle can be easily damaged. Clutch judder is usually easily solved by resetting the height of the clutch operating fingers.