can I replace it with one with a revcounter
No, regarding odo, will just show actual RPM.
And the adding of a rev counter, would I need to install anything for that or is it just plug-and-play into the old one?
You said you do not have a rev counter?!?
You have 2 options: 1. Electronic via ignition, minor cable wiring needed. Quite rare to find, many also function badly.
Mechanical wire driven , like 90`s SS models. You need an outlet in cylinder head, it`s driven via cam gear.
Not so easy getting parts for that.
Try find replacement speedo, easiest way out.