The formula for the XS head would be to set it up to match the engine combination. One of the best heads out there would be the Lilly Head,expensive but flat out performs heads with 3mm oversized valves by the slimmest of margins but goes to illustrate there's more to extracting power that shoving big valves in. One of the weakest links to the Xs head is the stock combustion chamber is the lack of squish area to enhance combustion efficiency, the head responds variably well to port volume reduction on the intake side with no loss of CFM's ( If done correctly). The D-shape on the intake side is option you might consider,this combination will yield positive gains across the flow curve if done correctly. I personally prefer the 750 virago valves cut down to XS stock dimension to utilize the thicker margins in helping reduce flow resistance as it enters the chamber.On the exhaust valve, Virago valve when cut down,slightly radius the margin a few degrees in at the top for more HP . Depending on the cam choice,watch the exhaust flow ratio(dependent on cam lift) and you'll want the intake flow to begin flexing if muscle right around .200 to limit reversion during overlap and to broaden the torque curve.(must work the floor ). Having access to a flow bench would help you greatly in educating you how the XS ticks and responds to port modifications. The XS1 head "1970" has a solid platform,great velocity,well round ports with few casting flaws and are far more consistent from port to port over the later heads. As I stated earlier,one key area of importance in yielding good power is the the exhaust port. It's the culprit in choking the true breathability of the Xs head. So many ways of porting it,makes if fun and challenging.
Having seen the 750 liner,there's no way I'd take the bore out to 81mm,there's just not enough meat left in preventing liner distortion under max load. I'd be looking into getting ductile iron liners,IMO.
A good performance book to purchase would be the Xs650 performance from Craig Weeks and if you don't wish to tackle the head yourself,then get the Lilly Head.