that's a sweet looking Rossin you've got there. and DA 7400 components! dunno much, but seems having lotta potential.
to my untrained eyes, the fork does indeed look "not original".
generally 'newer, more modern', not complimenting the beautiful lugged frame much, unfortunately. (that's just me)

the rusty freckles on the chrome—also hard to tell from the photos as is—don't look too bad, tho.
try rubbing using a handful of cooking foil ball dipped in vinegar. most of the time, it works like a magic.
if not removable in that way, that's a good sign of having to move onto the next level—such as OA etc.

btw, the saddle tip angle seems hung up on a pretty uncomfortable level, no?

what do i know... congrats on getting a beautiful frame!