The XJ900 engine will bolt right in...but there will be airbox, exhaust, and probably engine/fuel tank interference issues. I picked up a 900 with that idea, then did some serious measuring. :huh Do-able, but not easy....there would be lots of parts-massaging required: you'd have go with individual air filters and re-jet, as there's no way to get the 900 airbox into the 650 frame. The 900's oil cooler is just different enough that it's not a bolt-on, due to the cooler's frame mounting points being classic Japanese same-same but not same-same. The exhaust system might be be a minor can of worms too. The 900 uses Mikunis instead of the 650/750's Hitachis, so expect more fiddling there in ways than you might not expect... Let's just say that a 750 engine puts the 650's petcock noticeably closer to the valve cover. All the XJ650/700/750/900 engines bolted into the frame by attachment points below the cylinder base gasket, so all the engines will fit the same hole. The surrounding bits may not be so easy. I'm currently using a 750 Seca engine, XJ750 CDI box, 750 airbox, 650 Seca exhaust system. Had to shave the sidecovers a bit to get them to fit the slightly larger airbox, and both bikes share the same carbs with different jetting. I'm using slightly taller gearing, and it'll pull from 25 mph in top gear, never needs to downshift to pass, and the fuel mileage is the same as before. If you go this route, be aware that there are two different types of oil-level sensors in XJ-land, and choosing the wrong one for the swap will result in an oil light that only goes off when the engine stops.