Can't really help you there. But it would be a big undertaking, because the standard spindle as shown here
http://www.toolsandmods.com/mini-lathe-parts-drawings.html is 30mm diameter (1.181") within the headstock, so it obviously can't be bored out to 1.125". Thus, you will need to fashion an entirely new, fatter, spindle, which in turn will involve larger diameter bearings, and boring out the housings in the headstock to take them. The bores of the hi/lo gears hidden with the headstock will need to be bigger, as will the bore of the spindle gear.
You will also need chucks with 1.125" bores; the standard ones aren't that big.
Good luck!
Andy