MGOOF : Modern groupset on old frame

Sand the rust completely away and paint it black or so, there is no way to make it look like chrome again.

you could try one of these pens ...but it will probably look shite.

The only real way is to re-chrome (and loose the original paint in the process)

I never used it (for Tamiya models or otherwise), but maybe a little patch of this could work:

https://www.scalemodelshop.co.uk/300x160mm-ultra-bright-chrome-foil-self-adhesive-bare-metal-p7032

Even if it just stops your eye catching the black patch

Presume you've tried the aluminium foil cleaning technique?

In case, just checked, there is a 5 year warranty on frame and forks at cinelli.
Even if out of warranty, they might send you a replacement frame at a discount.

Rub it down with some scrunched up tinfoil covered in full fat Coke. Works a treat.