The six cylinder engines derived from the four cylinder engines that Offy built are accounted for and are in existence. One is in the Blue Crown Special that is owned by Dom McReynolds of AZ (its for sale)
http://fabulousracers.com/bluecrown.html and the other is owned by my friend Chet Knox of So Cal and is in the Justice brothers museum. About five years ago he wanted 25k for it, but like everything ells is probably gone up considerably.
What is even rarer is the stock 302 GMC head and the 12 port head that Offy worked on.