For a center in the spindle, you will need the spindle adapter sleeve that is of the right taper to fit and then a dead center that fits the adapter sleeve. I have not looked but in the PDF you posted it should list the spindle taper, The adapter is a short version of the morse taper adapters that one can buy from industrial supply houses.
Second on the belt. Do you want to change the belt or just move it from pulley to pulley? To change the belt either requires disassembly or cut the old belt and replace with a Fenner Powertwist Belt. I choose the Fenner option. Better in the long run. If just moving the belt you have to release the tension via the lever on the side of the head stock.
Pierre