I'm planning some modifications to my '93 K75 that will involve some frame welding. By my rough measurement (with calipers), the frame tubing OD is about 1.2", or 30mm. Has anyone done frame modifications on a K75 before, and can recommend or link to the specific steel tube stock they used? I'd like to use stock that's consistent with what's already being used in the stock frame if at all possible...

Ideal option is to acquire a spare frame if you are making substantial alterations - it comes with bends and everything! Failing that I would advise cold drawn seamless mild steel tubing - the sort of stuff that is required for roll cages. It can be had in various wall thicknesses, but a quick look on oohbay seems to show that it can't be had in 30mm OD. I used 31.75OD x 2.5mm for the two front downtubes on the Madass and that's been fine. I would recommend starting the cuts on your stock frame first, because that way you'll know the wall thickness you require - I found that wall thickness varied around the stock K1100 frame for example when I set about cutting one up for modification.

Long-time member Duck has spare frames in various stages of their life about his place on the Hood Canal, some of which may still be for sale.