If you can find any Westly's bleach white whitewall cleaner. And a Arm and Hammer Magic Eraser. I hear the new Westly's isn't as good as the old stuff. Found a gallon of the old stuff in dad's garage. Always used it on his old Cadillac's. I have yet to see anything work better.
Harley makes a good spray and wipes for tires and white walls. It smells like oranges so I'm guessing it's like ka-boom. It comes in black spray bottle. Works great, lifts the dirt right up. One wipe with clean towel and good as new.
The whitewalls on my Vintage were almost black due to extended neglect. They were so bad that I was too embarrassed to take before and after pix. So there ain't no way in hell I'm going to all black tires, except for maybe when I ever get off my ass and go on the bucket list ride to Alaska maybe. Anyway, I rode down to my buddy's house down the street who has the four Harleys. He has a shop full of the accessories to do everything too. We jacked it up to where the wheels could be turned and I cleaned them up pretty good. First I wet them down, then squirted on some of that toilet bowl Scrubbing Bubbles stuff and rubbed it in with the fingers. Then I scrubbed them with moistened Brillo Pads. Rinsed them off and they looked good as new! It took longer to jack the bike up than it was to clean one tire it was that easy. Used about one Brillo Pad per tire, and maybe a third of a bottle of the Scrubbing Bubble stuff. Most of that drizzled down on the ground while applying it. Give it a try guys and tell your friends about it too.