The Hoover Portable Vacuum has a disposable bag and 2 filters. Behind the bag is the pre-motor filter, it kind of looked like a thin hvac filter, the cardboard reinforced a thin fabric. The pre-motor filter is there to protect the motor just in case the bag breaks and to help even airflow out a little. I would look for a modern-day pre-motor filter foam and cut it to size. The second filter is a little foam by the exhaust, it was more of an air diffuser as both pre-motor and post-motor filters didn't really improve filtration back then. Again, I would cut some filter foam and fit it to the exhaust. If you can't find a good vacuum pre-motor filter, you can also use HVAC filters like this electrostatic media filter or carbon filter.

The bag is the main filtration. I would try to adapt a hepa bag to the dock because it will filter much better and provide much stronger protection against the bag breaking.

Ibaisiac made a great video on the Hoover Portable 2120 and you can see the pre-motor filter and post-motor filter location.

I don't think it's really practical or convenient to use a Hoover Portable today. You have a lot of great canister vacuums available in Europe: Numatic Henry €161, Bosch €119, and even Karcher Wet/Dry Vacuum €74 when you use the fleece bags.