freddean2006,
With ingenuity and some sort of adapter plates, you should be able to mount larger steppers.
Maybe the manufacture would provide a stepper upgrade so you don't have to discard parts and re engineer a new machine.
Having the larger steppers would enhance the axis speed however it is only one part of the total picture.
Milling metal quickly or efficiently will never be a strong point with one of these types of table top machines.
Plan on a day or two if you want to mill a watch out of any type of metal.
Even with a 60,000 rpm spindle the depth of cut with small tools will make it excruciatingly slow.
You will enjoy it as long as you don't have high speed production expectations.
Jeff...