Best way to get a feel for the market search ebay under sold listing for "Singer 20 Toy sewing machine". I didn't notice any free manuals in a quick search but if you get an idea how it works, you can see and understand what's missing or broken. No personal experience with toy machines but I am currently tinkering with a Willcox & Gibbs Chainstitcher which is the stitch made by the Singer 20. Good Luck.
Jon