Help converting 27" rear cassette to freewheel cog

So... Unintentionally (but happily) got myself into this project. I'm trying to help someone with this and I have so far realised that I will need a special tool to remove the cassette to begin with. However, I am also unsure of what cog size I will need.

The wheel is a "Rigid", is that enough info to go by? It'd be great to find a cheap cog - they do exist for 27" wheels, right?

My plan:

1. Remove cassette
   
2. Install freewheel cog
   
3. Chop chain to fit (or will we have to buy a new one?)
   
   

Any help/advice is highly appreciated! I hope I've not duplicated a topic, I search the forum and didn't this topic.

Thanks