It is cheaper to replace your counter sprocket than the rear, however it is a bit harder on the chain as it does have to go 'round the tighter radius.
There also may be a clearance issue with a 12t counter as it moves the chain physically (pins in links) closer to the crankcase. My KX5 had the case worn in an unusual spot between the countersprocket and the casewhen I got it. When I went from a 14t counter to a 13 I saw where the damage came from.
Probably not the answer you're looking for, just something to consider.
Tuck\o/