I just bought this bike off of Ebay (yes, i know) because my bike got stolen recently and I needed something cheap to get me around. So it comes in yesterday and I take it to the bikeshop to get my chain installed and what not. Fast forward to 20 minutes ago I go for my first test ride and it was not good to say the least. Im pedaling along and apply some back pressure to stop and whoops....there goes the cog. Now this is definatley my fault for not checking this out but I just figured that my "fixed gear" bike would work as such. The hub brand is "Zeus" so can I just pick up a standard english lockring? While its in its dismantled state could I pick up a IRO cog and lockring to put on there? Im just learning the mechanics of these bikes and I just want to make sure I get everything right. Thanks for any replies.

Heres my situation in a pictorial form.