I have a 2006 CRF 450. I need to replace the chain and sprockets. I want to make the bike more trail friendly. I ride a lot of tight single track through the woods in Utah. I am thinking of going 13 on front and 51 on back. Does anybody have any thoughts on this? Also, if i change the sprocket size, do I need to change from the stock chain length? I appreciate any thoughts or feedback. Thanks.