I took a quick look on ebay for CB750 starter switch,
http://motors.shop.ebay.com/i.html?_nkw=cb750+starter+button&_sacat=&_dmpt=Motorcycles_Parts_Accessories&_odkw=cb750+starter+switch&_osacat=0&bkBtn=&_trksid=p4506.m270.l1313and there's 3 of them out there. If you can start it with a screwdriver, all it's doing is closing the low voltage contacts for the solenoid. So if it's a real Honda part, cross reference it with the Honda repair manual (found elsewhere on this forum). At worst, you're out the cost of the button (less than $10 US + shipping), and if it doesn't work, put it back on ebay.
Good luck, and let us know how things turn out.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.