Usually the keys on most typewriters are friction fit on and come off pretty easily. The tougher part is that the slugs are soldered onto the typebars, so you'll need to remove them and swap them with a soldering arm. Even if you have the soldering tools, the more trying part is aligning them properly when putting them back on. Many old school shops have/had custom jigs made for properly aligning slugs when soldering them on.

If you don't have the tools, patience or facility, this is usually something best left to your favorite shop: https://site.xavier.edu/polt/typewriters/tw-repair.html

reply to u/fontinalispluma at https://old.reddit.com/r/typewriters/comments/1gb3dwc/change_the_keys_and_type_slugs/

https://www.ebay.com/itm/404818669654

Mini Typewriter Blow Torch Soldering Tool Alcohol Torch

https://groups.io/g/TYPEWRITERS/topic/soldering_type_slugs/82277130?p=

https://www.reddit.com/r/typewriters/comments/14szjp/decided_to_share_one_of_mine/

Description and tools for swapping out typewriter slugs by soldering.

https://imgur.com/a/typewriter-tools-sSBsL

https://new.reddit.com/r/typewriters/comments/8t6ym5/dvorak_typewriters/

reply to u/67comet at https://www.reddit.com/r/typewriters/comments/1e4hu0s/smith_corona_electra_120_ribbon_carrier_not_moving/

That piece is called a ribbon vibrator which moves the ribbon up and down. They generally operate on gravity and as a result they need to be clean and free from oil, gunk, hair, dust, etc. Usually they slide up and down freely. There's a colloquial saying in the typewriter space that "A typewriter isn't really broken unless it's clean and broken."

Occasionally ribbon vibrators can become bent which makes them inoperable and this can be remedied with some light forming (bending) with an appropriate screwdriver or needle nosed plier. You can search YouTube and you'll find a variety of videos for cleaning and forming these back into shape so that they slide cleanly.

As for your missing 1/! slug, it's unlikely that you'll find someone selling just the slug itself and then you'll need to solder it on perfectly and/or adjust it slightly with appropriate tools to get the right alignment. Far better is to check around with repair shops that might have the same or similar machines which they're parting out and then you could request one. Your best bet is to purchase the entire typebar and slug assembly from a donor machine which you can then swap out into your machine and skip the soldering. For this https://www.youtube.com/watch?v=pFMu6dUROGA can give you tips.

To find donor machines, try repair shops on this list: https://site.xavier.edu/polt/typewriters/tw-repair.html