This is a total shot in the dark, but maybe you could use a preload adjuster from another coil-sprung fork with the same stanchion size. Maybe even from a different manufacturer, since they would likely use the same thread. You could take a $10 gamble on a Suntour preload adjuster:
https://www.srsuntour.us/products/preload-dial-adjuster-1
You might also try eBay or AliExpress. Sometimes they have obscure odds and ends. Like this kit of Dart parts that includes a knob labeled "preload": https://www.aliexpress.us/item/3256805637110919.html?browser_id=8a1984772dfd4ab080c71915b949e34f&aff_platform=msite&m_page_id=adfdabcbef1900ad6c6a8111bffe272536d7dee6fc&pdp_npi=4%40dis%21USD%2129.48%2129.48%21%21%21213.18%21213.18%21%40210307c117181687661077207ed2ce%2112000034479042138%21sea%21US%210%21AB&algo_pvid=8def227a-db37-43af-a5e8-b289a18ad24f