Standard bore and stroke are both 86mm, and a good carb to run is a 39PWK(thats what i run) a lot of guys bore them out to a 41. As for the engine build it depends on how much power you want and the type of riding you plan on doing. I went for a dune bike, I like to race up the hills so I'm ported, run a fuel customs intake, Delta V-vorce3 reed cage, CT racing outlaw pipe, 39 PWK keihin carb, and bored 3.5 over which makes my bike a 541cc. For fueling i go 50/50 with pump and 110oct race fuel and my oil is mixed at 32-1 I like to use Golden Spectrum. It's a dune bike not a drag.