Sixty Fiver, thing is though that I'd like to get rid of that nasty cassette. That would have been a great option though.

Sillygolem, unfortunately I won't have access to the bike until about 1 week so I can't determine physically if it's a freewheel or freehub.
Since the bike is in Sweden and I will have about 2 days to spend on it before I have to get it back down to Paris I need to know before what kind of wheel it is so that I can order necessary parts etc.

I found really cheap single speed freewheels on velosolo.co.uk/shopsingle.html called Dicta Singlespeed Freewheel.
They're only about 10 bucks and might be a good start. I might just order a Andel Cro-Mo Singlespeed Cog just in case it's actually a freehub.

Would these work in case it's a freehub/freewheel?

H