I have one of those Suckitup. That was the small budget Kenmore canister from the 70's and early '80s. Use a NACE bag for a Numatic Henry/Robert/Hetta, etc.. It is a cloth HEPA bag that has a nice stretchy opening that fits the Kenmore spout perfectly. It is a large bag. There is also a Riccar/Simplicty HEPA bag with a stretchy opening that is smaller that might be a better choice for the small machines but I haven't tested that one yet. With the NACE bag you have to bend the cardboard bag end over at the bottom a little for the red bag adapter to seat fully.
Filter foam can be bought at hardware stores. I use the thin replacement filter foam for window air conditioners. You can also use some modern bulk filter media if it is thin enough.
The premium versions of this canister, the ones with the attachment tray cover, came with full sized two stage Lamb motors just like the full sized canisters have. In theory you could put a 5.4 motor in one and have a real powerhouse. I'm not sure if the little exhaust opening at the rear could handle the airflow.
I haven't had Rucksuckers success with Lamb motors however. About 3/4s of the old vacs I have with Lamb motors have needed rebuilding. One stage or two, it doesn't seem to matter, most have bearing noise. The most recent one sounds like a garbage disposal full of ice or bones. There is more than bad bearings going on in this one. I ran it for all of two seconds before shutting it down. I didn't want to scatter it and frag my patio, and me!
Prowl ebay. Hoses for these turn up pretty regularly. Any old Kenmore Powermate works with them. The one that came with it is pretty awful but the full sized Powermate works fine. It's a good little vacuum.