all 125cc motorcycles have 35mm forks

[jaybar6]

The xs650 has 35mm diameter forks. This is the standard on 125cc across all the manufacturers. As a replacement I do like the work of Cognito Moto in bring old motorcycles up to date. This picture is a XS with a R6 front end. Nice work!

But I think the forks of a cb1100ex would be a better choice. A more period looking front end. It has 41mm diameter tubes, a wheel diameter of 18” , better brakes etc. Jay

 

[Ratranger]

35mm was pretty much the standard back in the 70s-80s on import bikes. Maybe 1-2mm bigger or smaller, but all pretty close.

As for swaps you can do about anything. The early R6 and R6S forks have been used a lot since they are conventional rather than USD. Length will play a bigger role in a swap than diameter, a lot of newer forks are shorter so you either end up lowering the rear to match, or custom triples to gain some height back.

You also run into wheel compatibility, newer forks tend to have larger axles, which leaves fewer off the shelf options for spoked wheels.

Overall a fork swap can really change the look and feel of a bike, but if you can't make your own pieces it can get expensive.