yes, it has been done before and some say it works. however it may require some knowledge and precise machining. i have never done it before so i can´t help in details...
the big wheel and small wheel on the yz is the same suspension, so i guess it will work.
i have done this kinds of mods on some bikes and i never liked the feel of the bikes. simply, the rear suspension isn´t be mached with the front one. the yz forks are bigger than the ttr so the front will be very high, i believe...
what about do some mods to the crf to make it more rideable on thight stuff??