Holley 1920 missing parts - time to swap?

Disagree on both counts. The 1920, in working condition, is an adequate carburetor. It can be made somewhat less minimally adequate with careful time and attention, but it is not amazing. The main reason why Chrysler used it was that it was inexpensive.

Is it better than a BBS? By design, no, it's inferior. Performance? No, not ineherently; either carburetor can run better or worse than the other depending on the condition of the particular carburetor at hand.

But go with y'self; I don't try to dictate what flavour of ice cream anyone else ought to prefer, either.
I don't have any issues at all with the BBS, just stating what I have experienced personally, using both carbs back to back on the same engine, same day, same gas, etc.