Easily done! I'm sure we've all got a long list of 'beginners' mistakes' ! I've been collecting and renovating for six months and also started on an Underwood 5 - breaking a glass top was one of my beginner mistakes but since then I've become quite adept at removing key tops (and made a new glass top from an old piece of picture frame glass). Part of the enjoyment for me is puzzling out how to fix worn out (or missing) parts, and where to source alternative spares. The Internet has been a great source of things like rubber hose for paper guide rollers and replacement thumb screws for retaining Olivetti ribbon spools. In the UK, Hoseworld.com has proved a great source of a variety of different sizes of rubber tubing (they offer to send the rubber hose in plain packaging...to protect the interests of deviant typewriter renovators??)
Good luck with the Underwood and everything that follows.
Ian J