I am pretty sure with motors that size (0.58A), the PWM isn't worth the effort.
At best, the fan above saved a hair under 25% at low speed.  Much less savings at higher speeds.
Myself, I'll stick with simple power resistors until the motor is quite a bit more powerfull, maybe 3A?

Samoa's circuit is easy to build.  Works fine with the changes I told my friend to make (Schottky Flyback, and better mosfet).

You can build a "regulated" transformer.  Same as what you had, but a bigger cap.
Then feed the output to a low drop-out 7812 (TO-3 size, with heat sink), and a small cap and 1M resistor across the final output.
It is not efficient.  I wouldn't do it for a motor.
Really, a "12V" motor should handle 16V no problem.