Doesn't have to be a "big bore". Just needs to be higher compression. Like James said, 10.25:1 is not enough with that cam. Step up to either the 12:1 JE (you'd have to do the 366 because that's all they have besides standard bore) or get the Wiseco 11:1. The Wiseco is a nice piece, there is one in my Warrior. Running that compared to the JE means it's more pump gas friendly as well. And, I believe they offer it in your bore size. You may only need to run a hone through the cylinder (if you know how) to deglaze it, then put the new piston in there.
Don't have to clearance anything head wise, only the decomp tab on the either pull start models' rockers. Yours doesn't have that. Honestly, no one needs the decomp with this cam because it bleeds off so much cranking compression, you can pullstart it with ease (ask me how I know, lol).
Stock cam gear is fine, but, if you want to adjust the powerband, popping for Mega's expensive ass adjustable cam gear is the only way to do it. Get that, and you can degree in the cam for more retard or more advance. Is it worth it? Eh.... Do I run one, yes. Best if you have a cam degree kit to set it up anyway.
Running Mega's 252x2 cam would work well with the piston you already have. HD valve springs of any brand is a must with any cam, period. Even the Hotcams should really have them. Best to stick with a well known MFG, like Kibblewhite, for the valve springs. Stock valves are fine, and +1 aftermarket intake valve isn't worth it.
Strokers sound cool, but unless you are taking about boring AND stroking to reach or topple 450cc, don't waste your money. Best to have some higher revs and lower gearing, then to have slower revs and taller gearing. A fellow I used to use for machine work once told me, "smaller displacement quads want higher revs to get the tires spinning faster. A dirtbike of the same displacement wants lower revs to it won't spin the rear wheel out from under it." Turned out he was right (as per my experience).
