I'm assuming that this motor drives through a worm and wheel, or some other form of self-locking drive, so the motor only drives when there is a demand?
The circuit sounds incredibly crude, it isn't fit to call it a servo as that would imply some finesse and precision.
The motor will produce somewhat less than 50% rated torque, and will get hot because of the huge AC component. A DC motor isn't normally designed to minimise eddy losses in the core, because a DC supply doesn't generate eddy currents in the iron in the way that AC does. As your controller is feeding the armature a mixture of AC and DC, the motor will heat up due to eddy losses, but not produce a power output commensurate with that heating effect. There will probably be significant torque ripple, although in this application you might not be too bothered.
The 47R / 100n network is a snubber designed to protect the SCR's from high dv/dt which would damage them.
I think you could make a useful modification by converting your circuit to a half-controlled bridge rectifier, which would use your existing SCR's and drive circuitry and add two power diodes of similar rating to your SCR's. This would give you a full-wave rectified supply which causes less harmonic pollution of your supply and give a far better DC supply to your motor. A win-win situation.
You need to do some more investigation into your firing circuit to determine what range of delay angle control it has, but the principle will work.
