> It has already happened to me that using the update
> option the mirroring started from scratch, and I
> don't want this to happen (because of the large
> amount of data).

Update does not start from scratch (except if the mirror is canceled very
early.) Update asks the server has this file changed and downloads it only if
it has. This is quick (5 files/sec or better.)

Continue, assumes the current files are correct and only downloads those it
doesn't have. This will not fix your 403 files.

> I've also thought about editing the new.txt,

I don't know if httrack even looks at that. Changing the new.zip will mean a
complete redownload. Don't do it.