I had no idea what pen I should use for this plotter. Manual which I found was not a help here, as there is no information about how to use the device, it's focused on how to repair it. After search on Aliexpress I got a real headache, because you can find at least 11 different adapters / holders there (i.e. here).
I made some measurements of a pen grip on plotter to have a rough idea of what I'm looking for. I got lucky with this 3D model of a plotter pen, which I 3D printed and installed in the device. It was a fit, although grip was not able to pick up pen from tray due to the rough surface of the printed part. Nevertheless now I knew what to look for. I ordered this type of adapter, which should be a good start before designing my own solution.