Hello Alex,
You might find someone on eBay selling parts for these 342 cars.
Otherwise check out these links for a seller in Germany with whom I deal.
I am not sure she sends overseas now, because I get wheels (not for the 342) sent to a daughter in Germany.
https://www.modellbahnradsatz.de...er-modelle-von-maerklin/

https://www.modellbahnradsatz.de...er-modelle-von-maerklin/

Kimball

From what I can see the 342 wagon is late pre-war or early post war series. The wheels for these types were the blunt thick end original Type, D12 in Dia. The spares numbers for these were 700000 700010, disc and spoked when still in stock up to around the 1960's, since not made anymore. Marklin went to the pinpoint and small diameter rod types hence and these became the standard. There many versions starting from 700020, 700070, 700150 etc which are available, however would not be suitable. Best to find a scrap coach of the period with similar wheels and use those.

Alternatively the wheels are available from https://ttn-shop.com/marklin-part-list_342.php

As said most wheels can be cleaned, they are very robust, which is your best option.

David