Wills is quite right 66
All hail Jo Lucas, "Prince of Darkness"
It was meant to be a clarion call for "electric" bike lights
But "dying glowworm" would be more accurate
I've had some scary moments courtesy of Mr Lucas
When I finally got an MZ I couldn't believe how good the lights were
And they were only 6 volt
Japanese electrics were a revelation when they 1st appeared
And the majority were only 6 volt
They were far superior to most British electrics
The majority of bikes were fitted with Lucas equipment
Smaller bikes had Wipac or Miller
Most only worked intermittently
Maybe the 6 volt system on your SP can be revived
If not then maybe your generator is putting out more than 6 volts
(most do)
Then you can just change the regulator rectifier/coil/starter & other ancillaries
But they must have been OK when they were new
So there is always hope that the lights will be serviceable with some TLC & good lamps