I'm on a 2002 KX250 and had my flywheel cover off a couple weeks ago. The pictures you posted look stock; but mine was smooth all around. It looks like yours may have been drilled to lighten up. ?? But the holes are only on 1 side, so it may have been done to balance the motor. ?? I think I'd consider your flywheel weight as stock.
As far as getting a flywheel weight for your bike, I recommend holding off for a few months. Ride the bike a bit more and see if you really need it. When I first got my 2002 KX250 I thought I needed a flywheel weight, I held off (because I'm cheap) but now I'm used to the power and bike and can go anywhere. I feel I don't need the weight. Unless you're doing some real tight and slow single track you probably don't need it.
If you do buy a weight, I'd go with a 9oz. This will help with the bottom end and not effect the mid and top.