Most of these universal silencers have reducers to fit several diameters of pipe. If it was me I'd weld the reducers to the downpipe and use the clamp to fit the silencer with the extra silencer mounting bracket somewhere on the frame - that way you know it fits correctly and you can remove it without taking the whole exhaust off. 125 single don't vibrate too much but as Kano says keep an eye on tightness.
Yonks ago I ran a GS750 low rider chop and it had one of these universal silencers on it, made by Jama, and it never gave me any problems at all - it's a whole lot cheaper than buying a secondhand Coguar one on the Bay of Fools and ending up,with some junk..
Thank u Overbomber and the rest of everyone.think i will get the universal one as its going to be cheaper and plus the slash cut ones look good.
Don't forget to post a picture when its up and running, all the pictures posted so far are 500/600 of some sort, we should not forget we all started riding smaller bikes and many have fond memories of our youth - except me its so long ago I've forgot it