Brian,
My nephew has an '01 XC125 that I put a GasGas specific FMF on, with the stock messico silencer. Not sure if they still make it but if so I'm sure you could get it to fit with minor tweaking. The pipe on his bike was speced for an '03 and it went on the '01 no problem. As far as the YZ pipe, a guy from Sweden on the old Smackover site told me it was possible, but I never tried it. Maybe get a cheap used YZ pipe and see if its possible? I also have jetting specs I worked out with the 38mm carb, I have to dig them up.
I rebuilt the entire bike last year and it runs great, the kid goes like stink on it. Decent low end and really good midrange for a 125.