Many years ago, I put together this circle cutting contraption using a magnetic base, a Grainger gearmotor, and a toy train transformer. The torch tip was held by the machine, while the operator manually operated the cutting oxygen lever. On-off and variable speed were controlled by the transformer.
It worked fine, but if I did it again I would use heavier parts. Back then, all hobbyists had were oxy-acetylene torches. Today, a plasma torch in this device would do even better.