You wont be able to slow down first gear only , any sprocket changes will affect all 5 gears equally
If your looking to take away punch and do it cheaply , you can go from the 13 front to a 14 , that will be equivalent to about 3 teeth on the rear sprocket , but it will also bring your rear wheel forward quite a bit , and will affect how the bike turns and how much rear wheel traction
If your going to experiment with different sprocket sizes , then buy a 120 link chain and 2 master links for it (1 should come with it) , then you could go 1-2-3 or more teeth in the rear for more subtle changes , or as i mentioned go to a 14 in the front
Here is a link to the Gearing Calculator here on this site , when the page opens just click on the words
"Click Here To Try The Gearing Calculator"
http://www.crfsonly.com/forum/viewtopic.php?t=37723
Hope that helps !