Cool, that makes sense! Thanks!

So all I need then is a cog like this? I guess there's a risk that the size is not the same since this pre 1985 freewheel is, as I understand it, smaller than the later ones...

http://surlybikes.com/parts/fixed_cogs/

Apologies if I seem totally lost but simply want to make sure I order the right stuff.

I'm the editor of a website called Projekt (www.projektblog.info) and I will put up this bike project as a post there when I'm finished.
Will def also put up a link and recommendation to www.bikeforums.net, great help!