That's a great project, the Ranger tool set in 111mm. Unfortunately I don't know how to get 3mm bushings alone. There is an ebay seller in Germany who sells 2.25 and 2.5 bushings (with brass pins), you may enquire if they have 3mm as well. It's also possible to find SAK spare parts on Aliexpress but I wouldn't know which search terms to use for this. One last possibility (short of destroying an additional SAK) would be to peen your mod without bushings and glue the scales. Good luck and please post pictures of the finished project.
I'm not sure if you can name sellers here, but if you search for "2.25mm brass rivets" on ebay it should be on top of the listings.