That's well beyond small servos. 50cm long and 1kg is 5Nm of torque (50 kgf-cm of course).
Small geared down motor with integral encoder is one possibility, geared-down small stepper
is another. Gearing can mean toothed belt+sprockets, not just gearbox. Belts can also
be used to allow mounting of motors back at the arm pivot so they don't contribute to arm mass.