Having tested, replaced, renewed or re-soldered all the obvious things around the magneto, I have set about tracing connections 'upstream' towards the battery. I found that no volts were being sent to the magneto area at all, though the battery and fuse are fine. So I demounted and opened up the ignition switch and the contact plate came out in several pieces. This could be the problem, an old plastic part broken apart by vibration and subsequent loss of contact electrical flow. The whole ignition assembly is part number 2518250821, but no-one has one, even in the US, it seems. Though, in fact, all I need is the tail of the loom and the contact plate. I'm looking at whether it's possible to mend the existing part with epoxy if no spare is available. Anyone had or solved this problem? Anyone have a spare from a broken machine? All help welcome! (Pity photo attachments aren't working.) Can PM me on <This email address is being protected from spambots. You need JavaScript enabled to view it.>

What you found "no volts were being sent to the magneto area at all" is completely normal. 

The magneto is a "permanent magnet alternator" and does not use battery power at all for the ignition or the lighting/charging.

With regards to the battery specifically, there is a diode (aka "rectifier") between the magneto and the battery.  The diode allows positive voltage to flow from the magneto to the battery.  The diode is basically a "one way valve" so it prevents voltage from the battery flowing to the magneto. 

Bottom line is that if you found battery voltage at the magneto connections that would be a problem.  Not finding battery voltage at the magneto is what you want.

On your ignition switch, Yamaha had different versions especially in the 1970-71 era.  If you have a US model the switch may be much different than one from another part of the world.  Best way to get the correct one is to list the connection types and wire colors you have.  For example you might have a 4 wire plug (list the 4 colors) and 5 individual wires (List the 5 colors)

And also since there is often model confusion and parts swaps, it's important to know if the switch is mounted up on the steering head or down on the left side of the frame under the front of the fuel tank.  (CT1C should have switch up on steering head)

Hopefully someone in your country will have what you need...  if not I can look in my box of spare electrics and see what I might have. 

Also you can post photos to a hosting site like Imgur and link them here.

Maybe keep an eye out for clock set/speedo/tacho/bracket to which a main switch is still attached. These can be quite cheap, although the ones on fleabay at the moment are far too expensive

Hello - I know I'm late to this but I have an extra ignition switch with a 3 wire connector (red,brown,green).  Not sure what it goes on but I did have 2 new keys made for it.  Reasonable price!  
Regards, Joe