Repairing rotten exterior basement window sill

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?
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First, I'd look for the cause of the rotting and see about fixing it. We don't know what the cross section and dimensions of the sill are. If the section is simple and the piece long, I'd consider a new piece. The joints / connections at the end might be difficult to re work. New wood will not be as dense as the existing wood. I would use two part epoxy to coat the new piece after dry fitting it. TB
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