Lathe restoration SN20

[Toolmaker51]

Right off, I'd google for images of that model. Is the machine graduated metric or imperial? Equipment seemingly not so prevalent on one continent can be prominent elsewhere.
1] Count gear teeth outside the headstock, pitch of lead screw, and mark them with a paint pen.
2] Overall, you'll be more interested in feed and threading plaques than RPM. Feeds can be measured by a protractor and travel indicator, provided you can rotate spindle manually.
3] There I'd open an excel spreadsheet and place info recorded in cells. Calculations will be just division and multiplication.
4] Speeds can be determined numerous ways. Depending on electrical supply, Hz and voltage in particular, you can determine via motor tag at your power available.

I helped generate plaques once in this very process. With all info compiled, spreadsheet was artwork for brass plates to be photo acid etched; fonts, decimals, arrows and all. Brass plates can be thickness of shim stock on up. Surprisingly low cost compared to engraving, and realistic standing figures, vs those in relief. Either can backfill with paint highlighting. There are companies that will cast from a pattern, costly and lesser distinct detailing.