The goal of this bike is to be as light as possible. Big bore means weight. Plus, the are many tricks that have been utilized on this bike in livening up the bottom end.
--'03 CRF450r--
--FMF Pipe, Pro-Tech Revavle--
--Vortex Basket With Tusk Disks And HP Springs--
--Pro Taper Triple Clamps And Bars--
--Devol Full SP With Moose HGs--
--Renthal 13/50 With R1 Chain--
--Now It Is The End, Probably...--
--'06 CBR F4I--
--Bone Stock--
It would probly end up weighing the same. Since you take some material off the cylinder and add a little to the piston, if it doesnt equall out it would probly be like 1 gram heavier. But i wouldnt get the big bore for more bottom end, i would add to the 125s strength and get it ported for more midrange, and a little more up top. I would try to make it like a 250f as much as possible
2001 YZ-426f- Aftermarket radiators,radiator braces, rebuilt forks, new linkage bearings, new piston, wrapped header, 5th dragon graphics(or some shit), TAG tripple tree(Thanks TDB) and Turner OV bars.