The filter element has no spring clips, it just sits inside the housing, fairly tight against the perforated inner part. I assume that is how it’s supposed to be fitted. Isn’t the filter basically the same as on a 15/6? On Jose Franssen’s parts list for the 15/6 it shows a filter element that looks like a circular zig-zag arrangement but they don’t list a replacement element. Time for some ingenuity, I suppose.
The foam sheet in the pictures looks like it ought to work well. I imagine I could find something like that easily enough. The felt that I have is heavy with crud and is practically solid so I don’t think washing it with petrol will help.