you want to be careful going down teeth on the front sprocket. making the front sprocket too small can bind the chain. down one tooth in the front is the same as up two teeth on the rear IIRRC. most go down one on the front and +1,2,3, etc in the rear. down in front and up in rear will reduce top speed but increase the low end torque. i know for 03-06, the stock front sprocket is 16 tooth and the rear is 43. looks like in 07 the stock rear is 42 teeth. i have -1+2 on my '04 and i love the pull. -1, stock rear on my '03 and there is a noticeable difference. good luck. last i saw, motomummy had great deals on chain kits. always change both sprockets and chain at the same time. otherwise, you will experience premature wear. GL
Before you decide to make any changes ask yourself why are you thinking about such changes ? Would it be for fast take off from a dead stop , want to be a wheelie master , the mod factor or what .
General rule if you go 1 down in the front that equates to +2 in the back its usually cheaper to change the front sprocket & not the rear .
You will have to get a speedo healer to adjust the readings of the speedometer that if you want to have accurate speed readings while riding not that such reading are perfect in stock format as speedometer can be + or - 5 % from factory go figure.