whoa whoa whoa whoa, (sweet child of mine) hang on a moment. what are you trying to achieve here? why do you want to bore the barrel? if you have a big bore piston? is it right for your engine? why do you want to flow the head? do you understand these terms? are you just saying that you want to flow the head and bore the barrel because thats what you have heard you need to do to go faster?
the first thing you need to do before tuning an engine, any engine, is to understand why you are tuning it? do you want more power? do you want more torque? where in the rev range do you want it? do you understand the relationship between power and torque? have you factored in carburation exhaust and ignition requirements for the tuning you want to do? sorry to seem to bombard you with technical terminoligy, but everything in tuning engines is technical terminoligy, and you could well spend a fortune and end up with less power less torque and poor reliability. from your original post it appears that you havent got much of a clue about engine tuning, with that in mind are you really sure that you want to start throwing money at an engine tuning job when you dont understand how and why tuning works?
sorry if this sounds like im pissing on your bonfire, but even on a single like an xbr you can spend many hundreds and gain perhaps 5 or 6 bhp.
and im assuming you are fitting an xbr engine in its entirity into a cb250rs, cos from your post it sounds like you have a barrel and head that you plan to fit to an rs250!
before you spend a bean, tell us exactly what you have and exactly what you want to end up with,
