I too have been thinking about a replacement for the sponge and spring. I have used domestic cooker hood filter material which I glued together around a cylindrical mound (plastic pipe) It worked but I got fed up making replacements (due to a leaky exhaust which quickly mucked up the filter. So now I want a more solid replacement - like a vortex filter on performance cars. Problem is there is not enough space between plastic pipe and side of engine block so it will be necessary to have a bend and place the filter horizontally On my D2-55A which I think is the same as yours the outside diameter of the plastic pipe is 60mm (inside 45mm). I just happened to have lying around a 60mm plastic elbow with internal rubber gasket which should fit, the other end would take a 60mm internal diameter tube for the filter. The pipe was from a boiler balanced flue (inner pipe) It should not be that difficult to find a filter with that dimension. If you are going to follow this approach and identify a filter, I hope you will post the results.
