PSI sells a variable speed motor for mini lathes that runs about 100 bucks. You have to drill the mounting plate, but that might work in your situation.
The only downside to these motors I have heard of is one failure, and that the low speed is only 600 RPM where the mini lathes normally go to 500 RPM. Some guys feel it makes a difference in sanding. Maybe, but it's not that big of an increase. I would check out Penn State. See if what they have would work for you. At only 30 bucks more, you get variable speed. I'm just waiting for my motor to die but the darn thing just will not give up the ghost. 7 years old and runs strong. Darn it.
Al @ THO