About 1/3 of the length of one of our basement window sill has almost
fully rotted away (it is there in appearance but is as soft as paper).

The timbers are probably 100+ years and are dimensional.

What is the best way to fix?
- Cut out the entire sill and replace with new wood
- Cut out rotten part including a safety margin of good wood and
replace with new piece (perhaps with scarf joint) being careful
to make the transition as invisible as possible
- Replace the rotten area with Bondo (though warning it is about a
third of the entire sill)

Any other things to be careful about?
Any good tutorial sources on the Internet?