tradedog

Hi all. I have a Montesa Cota 311 which is is, as you know, a 2 stroke. I would like to convert it to a 4 stroke and was wondering whether anyone here has any ideas as to what motor might fit? I don't know whether there is enough space above the existing cylinder head for a 4 stroke head. Does anyone out there have any ideas? Maybe somebody who has done this before. I was thinking a crf 250 motor. I am not a trials competitor. I use this bike for cross country riding with some slow technical stuff. I know many people will say "stick with the 2 stroke". I just like 4 strokes and if I can do this conversion without to much difficulty I will try it. My thanks in advance.