There's nothing particulatry mysterious about a 283, other than the fact that the newest ones are over 50 years old, and finding one that hasn't been rebuilt a couple of times already is going to be difficult. Are you looking for a "numbers-correct" restoration, or just a reliable daily driver? Finding a set of 58 CC combustion chamber 305 cylinder heads from the late 1970's or early 80's will be a lot simpler than locating rebuildable 283 heads, and they're a bolt-on swap that will give you the correct compression ratio.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!