With the key, it depends on whether you mean just the key top or whether the key arm has broken off. If it is just the key top, you can pop one on taken from a parts machine. If the key arm has broken off, then that would be very challenging to replace.

In regard to the type slug, you could either solder on an new slug, or replace the whole typebar. Either way you will need a compatible donor machine with the same typeface in the same pitch. Soldering type slugs is not a beginner task.  Replacing the whole typebar is actually not too difficult on a 50's S-C, but the donor typebars can be out of alignment when swapping to another machine, and may require forming/peening to get in better alignment, which may be difficult without the skills and proper tools. Of the two options, I usually go for replacing the whole typebar, but that is because I can tweak alignment but I suck at soldering.