Yes, many people have opted to convert to cng and there is no modification of any sort. Its runs good too, you have to change nothing from what i know.
One member by the id of "Allee_Qazi" runs his mutt on cng and has posted here in the past. There are/may be that i don't of but i'm pretty sure there are plenty. I have occasionally seen mutts on cng in my local area, in fact most of them have opted for CNG and some have converted.