I'm no expert on that CDI but I am pretty sure that the original system didn't have a lighting coil.

Also no expert, but would be curious to know what happens to the lighting curcuit when running backwards with that much hp ........maybe the creation of a black hole?

    
here's some pictures of the stator and switch
it Got the Hitachi cdi box I don't know much about these bits but looks original?  

That maybe a DT360 ignition. the SC didn't have a lighting coil. Those aren't always dependable, if you plan on keeping I would suggest the VAPE ignition. 
What's the engine number?
Should be 363xxxxxx
 

Horn button looks original and standard fitment (according to CMSNL).

Light switch looks like one of the YHA205 or YHA206 variants (this number will be cast on the underside).

For the time being, I might be tempted to swap over those switches for one that includes the lights, horn and indicators (any one off a RD, XS, DT etc would do the job). Keep the original horn button, though, in case you or a subsequent owner decides to put it back to original spec.

from sunny Berkshire.
If you haven't got the original title document from the US you'll probably need to send a letter with your DVLA application stating why you don't have it.
I registered a couple of CT1's last year and both applications were sent back, but once I had written a brief explanation letter and sent everything back I received my documents quickly.
Here's a link for reference www.yamaha-enduros.com/index.php/forums/...641-uk-dating-letter
All the best.
Pete
 

Thanks for advice Mark,I have ordered a new electronic ignition so hoping that will work. Engine does start 363

Yes, I agree it looks like a kill button, not a horn button.

I also agree CDI looks original.  Parts list shows the 1974 model (SC500A) had an extra stator coil the 1973 model didn't have.  They call the extra coil "Pulser 2" and it looks a little like a lighting coil.  From what I can tell, Yamaha added this second "Pulser" in 1974 to correct issues with hard starting and poor low speed performance.  Service Bulletin M4-032 talks about the new stator and also retrofitting it to the 1973 model.

It would be interesting to see where the wires on that light switch on the handlebar are connected...  and/or if there is an extra wire coming out of the stator that might be from a lighting coil? 

 

Thanks Pete, I will give it ago and post updates when I have them. Regards Simon