Look through the "custom 2 stroke" section on everything2stroke. A guy has a YZ250 motor in a YFZ. It's a very clean install and he says it rips. Maybe he can give you more detailed info.
anything is possible with a welder,cutter and grinder...
i remember back in the day(250r days) dudes use to run the open class with cr250 motors in r frames.they use to tell me that the dirtbike motors produced more useable power then the 250r motor(whatever that ment).hell,try it out.you might just build a mx monster..