Any idea on how to add a rear brake to my Fito Modena GT? All I have is a disc brake in the front and coaster brakes

[JoeMetro]

There's no way I'll get a disc brake on the back because the sprocket is in the way, and I'm pretty sure the rim isn't made for rim brakes.

I'm thinking about upgrading the front brake to hydraulic, but I'm worried that tip over forward when braking hard because all the stopping power will be in the front.