Hello,
I have A Curta II SN:558860 with the plastic type clearing leaver, which was damaged. I now have a replacement.

To fit it, does it just snap onto the pin (it seems like a tight fit and I don't want to break the replacement) or is there another way to fit it, is it something I can do myself?

The plastic ones are snap-on, snap-off. I'll sell you a replacement for a reasonable price (a few bucks) if you manage to break the one you have on installation, so go for it.