In case you want to save as much original parts as possible, the old stanchoins can be saved. The tubes will be straighten, the rust pittings and mines turned away and then rechromed to original diameter, (with a thicker hard chrome layer). There must be company's in the US that do this when we have them.
Or look up for complete front ends (forks with triple clamps) that are similar in fork length from likewise bikes of Yamaha. So you don't have to care about the diameter of the forks. In my experience most fronts end are interchangeable.
I mounted to my Yamaha TY 175 twinshock (modded to 200ccm) now the front end of a TY mono 250cc which was not so difficult, I just needed different bearings and an extra mount for the drum brake arm. (The frame is a NOS British Mick Whitlock mini Majesty frame from the beginning eighties, saves many pounds in weight).