I'm not sure exactly where it states requirements but as an example
the FAA requires kits to have 51% completed by the owner. They just
put a moratorium on new kits approved for the 51% rule and are trying
to evaluate programs like Lancair and other "weekend" kits. This is of
course for Experimental-Amateur Built. You can probably register a US
factory built glider as exhibition or racing and get away with it. You
just wouldn't get the repairman certificate for having built it. I
think this is how some aerobatic aircraft like the Zivko Edge do it.

http://www.eaa.org/news/2008/2008-02-15_kits.asp

You can't register something as ELSA or SLSA that has already been
certified, so you can't convert and do maintenance on a 1-26 or Ka-6
that already exists. If someone wants to start building them again
like they've done with the Cub and Champ then you could.