I have a FG with a flip-flop wheel that now only has a fixed cog on one side, and I am looking to get a freewheel cog on the other side but I have a few questions first. Am I safe in assuming the freewheel mechanisim is in the cog itself and not in the hub? Is a lock ring required? Are they only one size, the threads I mean? Will a BMX freewheel my hub now? Thanks.

Buy a freewheel screw it on and go ride. It's that easy, really. A BMX freewheel will work.

With the exception of some french and some weird english things, most freewheels will fit EITHER side w/o a lockring (don't need one with freewheel).

BMX freewheels will work fine. A lot of people use 'em.

Thanks for the info. Two more questions; Is a secial tool needed to put it on or take it off, will a chain whip work? What's a ballpark price at the LBS for a BMX freewheel?

A freehwheel just screws on. You'll need a tool to get it off,but why would you need to? shimano BMW free wheels are like 15 bucks.

Yeah, $15-20. You'll need a freewheel remover to get it off, like this:

https://cgi.ebay.com/PARK-TOOL-FR-6-B...em280094614785