I can offer guide to 'How quiet the SL125 was when new'.
Back when the bike was new a friend had one, and got it stuck in some very soft ground in Wales. We took perhaps 20 minutes struggling to get it out, and then, as it was put on its side stand om firm ground, there was a cry of astonishment. 'Well I'll be blowed – the jolly little device is still running'.
So yes, it was a really quiet bike at tickover at least, when new.
Cheers, Tim