There are many variables. First you will need a modified crank with longer stroke. If you have the knowledge and tools to do that yourself, maybe you wouldn't ask these questions. So you'll have to buy one new, or try to score a second hand one and rebuild it. Either way, finding a supplier won't be very easy. Cranks that were used in motocross often have a turned down left taper because the charging system was swapped out for a total loss ignition like a Kröber. Then you'll have to repair the taper as well.
The big end pin diameter on your crank must match your conrod (either 256, 447 or 533 type). However, the larger 533 big end only allows very limited stroke increase, so you will most certainly end up with a 256/447 type pin.
There are 3 different length conrods for the XS, of which the 256 rod is almost unobtanium now and the 533.. see above. Apart from the remaining 447 rod, there are some additional 'long rod' options from other engines as well, but probably your pistons of choice will call for shorter ones if you don't want to end up with a very tall engine. Of course, if you have deep pockets, you can have your rods custom made by Carrillo or such.
Then you must determine deck height for available pistons in your targeted bore size, or decide to have custom pistons made.
Your stroke/con rod length/piston deck height will determine the thickness of an eventual foot plate. If its thickness does not end up being an integer multiple of the cam chain pitch, you'll need to rephase the camshaft by moving the sprocket.
At this point, you have a good mechanical fit, but an increased swept volume. You will have to adapt combustion chamber volume, either by modiying the cylinder head or piston dome volume, to achieve an acceptable compression ratio.
With all that said, the 'standard' recipe is an 84mm stroked crank, 256 rods, XT500 pistons, no additional foot plate. Which gives you the aforementioned 998cc and a decent compression ratio. You'll end up with an engine like in the 1010cc racer thread.
If you want to modify stroke based on the 80mm BBK, do the above homework and maybe you'll find something that works.
Make sure to let us know the outcome.