Just out curiosity to see what is left off the 11HP the bike should have, put her on the dyno.
As you can see on the original graphic, it should peek at around the 5000rpm
But from mileage of the engine I suspect that it has lost a bid.

Here you have her just after some runs. (catching her breath)

And here you have here results, axially not that bad at all. (considering she is almost 60years old)
Its a massive 9HP measured at the rear wheel. Peeking at 4960RPM

And how does that sounds?
Like this:

After the dyno on my way home, she started to give misfires.
Not alarming, but back home I checked the plug (haven't checked it before)
The plug is completely worn, have replaced it with a new one and it much better