Both my bikes have 175psi +/-. one has had the squish machined. I wouldn't think the 300 would be much different. I don't know if there is a published specification for the compression ratio.
You could probably use the clutch slave cylinder from a bike engine. Watch ebay as they get parted out often.
There aren't near as many ATVers on this site. Have patience and others will eventually chime in 
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jeff
'00 XC250 (mine)
LTS front and rear suspension, power valve cover, carb jet kit and rear chain glide, RB Designs head mod, BRP tall triple clamp, backyard foot-peg lowering conversion.
'04 EC250 (the kids)
(he can do his own mods haha!)
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