I acquired an ST2 engine some years back. I did so for the cams which I used on my Monster and then sold on most of it to recoup the costs (turns out I ended up getting the cams for 'free'). I sold the bottom end (minus side cases) to a guy in Ireland who was going to use it to replace the bottom end of an M900 that had suffered broken cases from a chain break. Looking at it there was very little difference between the 2 engines, other than the longer camshaft drive/layshaft for the water pump which could be cut down, replaced or (probably) just left as is if it fits in the Monster side cover. ST2 engine is 944cc but only by virtue of a +2mm bore, stroke is the same so 900cc barrels and pistons would fit perfectly.
900 barrels will fit straight onto the ST2 crankcases. The stud spacing is exactly the same. I fitted ST2 barrels and pistons (94mm) to my 907ie (94mm) a few years ago.
