the plastic piece
Thanks for posting. That plastic block is called a pivot shoe. I am sure we can replace it, but there's many different kinds, and the only way to know for sure which you need is to remove the old one and measure the width and thickness. Give us those exact measurements, and we can find what you need. See the video below that shows how to remove and replace shoes.
That said, the problem you are describing (the window won't stay up) sounds like less of an issue with the pivot shoes and more that the spiral balances have lost tension. The spiral balances are the rods with spirals connected to the pivot shoe. To see if that's the issue, first try charging the balances with a spiral balance charging tool like the 75-006. The guide below shows how to do this. If that doesn't work, then the balances probably need to be replaced to. See the video below that gives you an idea of the information we need to find the replacement.
