There might be a lens available to fit, but instead of the CFL bulb you have in there now, you could switch it for an LED floodlight bulb, which are meant to be bare, and will give you a much nicer light.

You can see two springs that hook on the trim ring.

Undo those screws. You can them measure the diameter of the metal can that is in the ceiling, or just take your trim to a box store. Go to the recessed light section and look for a "trim kit" matched to the the diameter of your recessed can.

Some trim kits are held by springs, as yours is. Some are friction-fit. You can use either.

You can kill two stones with one bird by selecting an LED retrofit bulb that will fit that can. Many of the bulbs are integral with the trim ring, the diffuser disc, and the spring legs that hold it in the can. Many have a screw-in edison adaptor in the package. Often times the retrofit LED bulb with integral trim kit will be comparable or lower in price than just buying a trim kit. The one I linked to has the spring-wire legs that hook into slots in the interior of the can. If yours has no slots, other retrofit lights have three somewhat stiffer spring legs that friction fit inside the can.