Most of the rebuild kits I've sourced (elsewhere) have had #62 or #64 mains. Recently, I got one with #66(!) which was way too big. These machines were jetting on the lean side, back in the era when you could get real gasoline. Fast forward to 2017 and oxygenated fuel and it seems that most do quite well with #62, plus/minus one size.
FYI, stock-from-the-factory mains ranged from #58 to #64. I've yet to run across a CT70 carb with #64...only a couple of Daxes, from the early `70s. No idea why there'd be that much jetting difference; they used the same carburetors.
The bottom line is that it's really up to you to learn how to dial-in the main jet size and jet-needle setting. Reformulated/oxygenated fuel has change the game a little. That most often means going 1-2 sizes bigger on the main jet, but not in every case. I'll second smitfire's recommendation and at least try the #58 first. If it runs properly, leave well enough alone. Main jets don't really wear out, or oxidize. Brass is very stable.
