Here's another possibility:
1x High torque RC car motor
1x RC car gearbox (optional for lower speed and higher torque)
1x Pulley (custom fit to motor or gearbox shaft)
1x RC car proportional speed controller
1x RC car receiver/electronics box (might have speed controller onboard)
1x Inexpensive 2-ch proportional RC transmitter
1x Slightly modified 120VAC to 9-15 VDC (power supply (similar to a cell phone or radio charger)
With the exception of the pulley and the power supply all of the parts should be available at your local hobby shop or from a myriad of online shops. Just make sure that your speed controller is adequate for your motor choice. It might also be a little more inexpensive (not to mention easier) to buy a complete RC car kit and just use what you need out of it. The power supply can be bought from your local electronics or hardware store and the DC voltage and current should match the requirements of your electronics box, etc. Mounting is left to your imagination.
If the object is to simply move a paper target back and forth, it just might have the guts for it. Also, to go with the latest comments, the compact nature of the components limits unwanted target aspect.
Just a thought,
Mike