Hmm i would choose the yoshi rs2 i am pretty sure the rs2 is for crf 50 but you can check it out on DHZ site. but out of the fmf and pro circuit probably the pro circuit if you were to get the fmf go for the 4.1 otherwise my opinion is the Yoshimura
If your going to leave it stock I would look at a pipe with a smalller header , id look at either , the fmf , fast 50s , drd , they have a smaller header & more suited to the stock engine compared to the pipes with larger headers.
I agree with Jase on that...yoshis look awesome and sound awesome but you will probably loose power. FMF and Fast50s are the only pipes that work best on a stocker, iv heard Dr.D pipes go hard aswell