Suggestions on HP21 restoration

[Marcovecchio]

I would like to ask for suggestions on how to restore an HP21 which has ruined electronics, but great housing and keys. I plan to use an Arduino (or similar) and a dot matrix OLED display to replace the guts.

I know the obvious way is to reproduce HP21 behaviour, which is not too hard. However, the hardware I plan to use will be capable of much more, and the LCD will also be able to display much richer info, so I would like to ask for ideas on what I can improve on the standard HP21 functionality, taking advantage of the powerful hardware.

My main concern lies on the keys: I want to respect key markings / functions. For example: HP21 has only one shift key. I can't add a second shift key, because I simply don't have any spare keys. All I have are the original HP21 keys, with their original markings/functions. This limits my possibilities, and I'm out of ideas on how to deal with that.

That would be perfect to, for example, turn it into a 42s, 34c, or 15c, but the HP21 keyboard won't fit these models. Any ideas on how to handle that?

Thanks in advance!