A 10% drop in voltage is still 226 volts which is still a higher voltage than 208 would deliver. A motor run undervoltage will in fact run at the rated speed as motor speed is related to the number of poles and frequency of the supply. Universal motors are voltage sensitive for speed but not a squirrel cage.
Your problem is going to be reduced torque and horse power. If the motor was oversized and the torque requirements over sized then maybe the motor will hum along just fine. How much the motor needs to be oversized is unknown to me.
Another issues is going to be efficiency. So if your belt sander runs 1000 grit on balsa you might be ok but if there is any 120 grit or courser and anyone pushes hard on the belt the motor will run hotter because it will run higher than rated current and will not meet your production or performance requirements. The cost of this motor might be very much higher than the cost of getting the right motor installled. Add it up. reduced efficiency, lower Hp = slower sanding. More electrons to pay for too. Cost of replacement before amortizing the purchase cost. Down time when the motor smokes at a high demand moment in production, etc.
Get a new motor. A boost transformer would work but you are again adding to the complexity and cost of what is probably not that expensive a motor to begin with.
