Brake light kit
It'll be a whole lot easier to build it yourself. I've seen cheap UGLY tail light kits on ebay that I wouldn't mount on anything.
They look like cheap trash which they are. Most bicycle accessories will vibrate loose, not operate properly, and break when very lightly bumped when installed on a motorized bicycle.
The brake light switch that I use was salvaged from one of those 7 function brake/tail signal light units that you can see on ebay.
The electromagnetic interference generated by the bicycle engines CDI, and spark plug wreaks havoc with most non shielded electronic accessories designed for bicycles.
My 7 function LED brake/tail, signal light now sits in a box on a shelf having been deprived of the only part that will function on a motorized bike. The cable mount brake light switch.
If you look hard enough you can see the brake light switch in the photo. It is on the rear brake cable between the brake shoe arms, and its adjusted by the same type stop that your clutch cable uses.
I assume that you care what your stuff looks like, so you'll have to build it because it aint sold for bicycles (yet).
J.C. Whitney sells lights that you can use. I included a good picture from JC Whitney for yall to see. They come in dual filament, and single filament.
All you'll need is something that uses a dual filament bulb.
Buy a switch that can be used for the brake light. Wire one filament to the headlight switch, wire the other filament to the brake light switch. Depending on what light you buy, You may have a third ground wire, if not it uses a negative chassis ground. Check out the light I bought on ebay. J.C. Whitney sells them too. They're called "accent lights" or auxcillary lights.