A lot of these trucks were brought into the US in that era. For filters, look in at industrial duty filter makers like Baldwin , Wix and a couple of others I can't think of at the moment. Pulling numbers off the old parts helps too.

It would be better to stop by a Freightliner dealer, ( if possible an older established one that sold these trucks in the day ) if you call at a busy time they are likely to just say they don't have any records. Having a pic and getting them involved in the fun of getting this truck going can produce results.