About 8-10hp difference between the two motors. A lot of it depends on how it is jetted and if the air box is gutted. If you just want to buy a motor and ride it, then get the 1000. You can lightly port the 1000 and add pipes and have 185-190hp all day long and put a lot of safe miles on it. The problem with the 1000's over the 900's for modding is that the 1000 is about a 1/2mm shorter stroke and it has a 22mm wrist pin which limits your piston options. The 900 motor has the 1/2mm longer stroke and a 20mm wrist pin so there are more piston options. Until recently a lot of engine big bore builders have not had good luck making good hp out of the 1000 cylinders (Transer ports ect are a bit different in size/angle than the 800 & 900 cylinders). My Buddy Ron Hunyady in Clio, MI makes very good hp out of triple big bore kits in trail form and he just finished up testing his first 1107 made out of the 1000 top end (He has built and fixed many stock cylinder and aftermarket cylinder big bore triples) and it runs VERY well. His flatland big bore kits are 230+hp on pump gas.
