While i too like things original, these dome parts are very pricey.
i might be inclined, too, to make do with another brass or gold lgb whistle.
that set and top “dome plug” disc may be impossible to find if marklin wont supply.
i would think making a plastic disc, 2 brass rods as pop valves, and a whistle would be a close approximation.
update after the above post,
otoh, i did look on ebay, to my surprise , everything is available, cheaply, say $4-6 US., including to my chagrin, the dome disc, but the shipping charges are laughable ($48 each)
good luck.