It's hard to beat a gearmotor for $/oz-in in low speed, high-torque applications. A 12V DC motor, 12VDC wall transformer, a pot and a DPDT switch should be all you need for speed and direction control. Check surplus sites for the gear motor or take apart an old whirring toy.
1200 rpm AC motor with a 300:1 gearhead should cost you about $80. 2" or 3" frame should be large enough. Direct couple motor to the load. The friction of the gearing should dampen OK during startup. If not use a lossy coupler like a section of stiff hose and hose clamps.
Being a motor guy, I keep reminding myself that insulation, bearings and air gap (and magnets if used) make a motor. If it were me, if oxide is getting inside the motor, then I would imagine failure is close. If the oxidation is heavy, but over the lamination, there is likely still a lot of...