I'm just gonna throw my opinion out there to try to help so just take it for nothing more than that. But I think you might have a little more interest in a lower compression ratio and a smaller bore size. I say that because 11 to 1 on an air cooled motor isn't a good idea on pump gas. Especially in the ES. On a summer day working and air cooled bike with 11 to 1 on 91 octain just isn't going to work. And I think 1 or .5mm would be a better choice. I doubt many 4 strokes are on their 2nd and 3rd bore and I wouldn't want to waste my extra bores. By that I mean when boring a cylinder I like to go the least amount possible and it still clean up.
And totally off topic, is it possible to get the 250X head on a 250SX?
08 Outlaw 525s
07 CRF70
04 Harley FLHRCI
04 TRX90
03 XR650R
03 400EX- XR650R engine
02 XR100R
01 Banshee
00 XR50R
00 XR70R
99 PW50
99 JR50
96,95 Sportsman 400
95 Xplorer 400
94 Indy XLT
87 Trail Boss
86,86,86,86,86 Scrambler
86 TRX70 x2
85 ATC350X
85,84 ATC250R
85 Red Tecate
85 LT50 x2
84 XR500R
84,84,84,84,83,82,80,79 ATC70
82,79 XL500R
83 ALT50
79,78,77,77,76 XT/TT500
76,75,74 XL70
72,71,71,71 SL70
72 Harley FX
72,71,70,70,70,70,70 CT70H
72 SL100
70,69 Z50