What type of bearings does it currently use in the spindle and if possible what's the bearing number (or dimensions)? It seems like a lot of work to make a whole new spindle if you could just replace them with tapered roller bearings, if it's not already got them. Although depending on how much faster you want to go and the size of the bearings taperered roller bearings may not be suitable. I would want to keep the gearbox, or at least have something in between the motor and spindle to change the ratio, as with a VFD (or servo to a lesser extent) you'll be lacking torque at low rpm. Very important for big drills, cutting stainless...etc.

It is important to keep the swarf of the ballscrew, perhaps something like machine bellows over the bed would do it?