Don't ride that bike until you have a front brake on it or you could be injured badly. Your front brake provides MOST of the stopping power.
The Weinmann 610 caliper should be fine, though occasionally the reach can be marginal even with 27" wheels. The Weinmann 750 would be better. Either way make sure you have some new pads on it.
You should have the hubs and crank serviced (lubed) as well, or you risk damaging the bearing surfaces.

Based on the decals your bike is a '73 or 74 model.
It's an OK riding bike (it was popular for good reason) but will be markedly better if you get around to replacing the steel rims with alloy rims. (It will stop better too.)
The Allvit rear derailleur is not very well regarded. A Suntour dbl parallelogram derailleur would be a great substitute.