2k is very cheap for a xb9 mine had 25k on the clock and I paid nearly 3. But to answer your questions Buells crankshafts are what decided the displacement a xb9 has a shorter throw crank but xb9/xb12 share the same bore. Without boring the case the biggest you can get a xb9 is a 1050 kit which isnt actually 1050cc i think its closer to 1015 or so iirc.
But xb9's are very fun bikes, revs quicker than a xb12 and for 2k at those low miles that's a steal.