You can probably repair the damage using the broken off piece, or a similarly shaped replacement piece, by reassembling the pieces, filling any small gaps with thickened epoxy, and then scraping/sanding it smooth after the epoxy has cured.  If it's along a long edge, the plans probably call for a fillet and glass tape on the inside, and glass on the outside -- which will make it plenty strong.  If you can make the pieces fit together closely, it will not look bad either.

Yes, you should have enough room left to lay out the pieces without encountering the messed-up scarfing spot. It's been awhile since I did that, but I remember having some wiggle room. (I had a knot in my plywood that I successfully avoided). You might need to experiment a little, but it it should work fine.    Even so, it's not a bad idea to patch the scarf together with the missing piece, if you still have it, or fill it flat with a woodflour/epoxy mix. If nothing else, that will be good practice for the future when you need to patch something up.

 Also, remember that  you will have an "inside" and an "outside" to the boat, so you can always figure on having the less attractive side of an individual panel (including its scarf joint) be the inside, where it won't be visible. (and I find it helpful to take a pencil and write on the inside side of the panel the words "inside" and also to mark which end will be the bow.....If you make the inside/outside decisions and write identifiying marks on the panels shortly after you cut them out, you can save yourself some anxiety later when it comes time to start beveling....and a little head scratching when it comes time to wire the panels.

 Julie K  (currently working on boat #7 -- a WD 12 from plans)

Thanks guys & gals.

Like I said, this is my first build and now that I've figured out what a great resource this forum is I foresee getting a lot more help from everyone. Hopefully someday I will gain enough experience and knowledge to return the favor!