Salt water usually does a pretty good job of messing up alloy. Maybe boiling the parts there-in to speed up the process?

Vinegar will probably etch into it too. Or caustic drain cleaner rubbed on with a rag and watched carefully?

Bantam Racer? Classic. My first bike was a 150cc D3 Bantam with sprung heel. Snapped I don't know how many front axles jumping it over dirt mounds in the bush down the the back of home. Great little machine. And some of them were tuned up to do phenomenal speeds (Not mine though!),

Got pics?

As a rebuilder and restorer of British bikes I see a case of over thinking this issue.

The relevant parts are 6082T6 aluminium. You do not need to do anything at all. Simply make the parts and fit them. The aluminium will oxidise naturally and as a result will go dull and become a natural grey colour. Anodising is best discounted as that will provide an anodic surface to the aluminium which will retain its clean silver appearance for years. Other processes like blasting, rumbling etc will also create a non natural finish which will for a long time look like the cleaning process took place. If you want to speed up the process make the parts and simply leave them outside over winter, otherwise let nature take it course.

What would happen if it was placed in a poly bag filled with oxygen ? Would brass tarnish overnight ?