Hi mate, that`s a good looking project you`ve got there! It looks to me just like a `57 gold flash with a `62 super rocket engine as you said, although if you came across a picky rivet counter, you might be told the rear mudguard brackets are the one piece arrow head type introduced in 1960, and the fuel tank, i think is slightly earlier as it may have had a round badge tank in 1957, but then i believe certain export bikes had the winged badge on in`57. Someone will probably prove me wrong on this but either way, as far as restoring a `57 flash with a `62 rocket motor goes, that is an enviable task you have there, because it`s complete and doesn`t look like it`s been standing around for too long.
Good luck and happy restoring!
Richie