It's probably a mis-print (unless RMATV is having these specially made for them, which I doubt)
The biggest piston that I am aware of is 89mm and the cylinder has to be sleeved to install it. If it were possible to go to 94.5mm, the cases would have to be machined for the larger sleeve and that would get into the bolt holes that hold the cylinder down.
In my experience it is not possible to take the 400 to 491cc unless you were starting with at least a 450
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