YZ 400 Rebuild Top End Advice/Cylinder type

I'm rebuilding my top end and would like advice/links for do's and don'ts.

I've rebuilt 2 strokes, read the manual on my bike, and know the basic routine, but this is my first 4 stroke and I'm not sure if I should keep it the stock bore, or go up (reliabilty key for me) and particularly which cylinder I have (wiseco shows 2 different pistons for my bike). Quickest/Cheapest piston delivery? What is the "rule of thumb" for replacing cyliders (I've heard anything you can catch a nail on should be bored out).

I understand theres tons of info I'm asking, so sorry in advance, I just need a good starting point and to make sure I'm on the right track.

The reason I'm doing the rebuilt was a blown head gasket.

Any help would be great and thank in advance.

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