Any single-speed freewheel would work. I recommend shimano mx-30's for an affordable, reliable option.

You don't need a tool to install a freewheel, but you do need one to remove it. Park's FR-6 will do the job for most BMX freewheels. To install, remove the wheel, grease the hub threads on the side of the axle opposite the currently-installed fixed cog, spin the freewheel onto the threads, and flip the wheel. If you go with the same tooth count as the fixed cog, you don't even need to break the chain.

A higher tooth count in the rear will make for an easier time pedaling, at the cost of max speed. It looks like the wheel axle is already pretty far forward in the track ends though, which may prevent you from being able to install a larger freewheel. If you do go bigger, you may need a longer chain as well.

Make sure a rear brake is installed when going single speed. It's much better to have it and not need it than to need it and not have it.