Probably best to do the exhaust mod then go for a run and check your plug.
My Agility 50 was running rich according to my plug so I ended up drilling 6x holes about 9-10mm each in the front of my stock airbox near the bottom. Tried that and it was then too lean so mess around with duct tape over some holes and found the 3x holes open and 3x taped over works fine, plug now reads perfect. If I had thought about it longer I would just have drilled 1 hole at a time till it was right, but I didnt. Plan on upgrading to an open filter eventually anyway but this works for now.