This is just a guess but it might be the cam stack
https://www.sewingpartsonline.com/pa...g-machine.aspx . The cam stack is the piece that allows the machine to make different stitches.
Any plastic parts in your machine are on their last stitches. I too had to make the decision to not fix my machine I bought in 1980. If you check my signature it's listed as RIP Singer 1036 because it was difficult to say good bye to my old friend.