Welcome to the forums. Yes, like stated above, the seal can be changed without taking the engine apart. Just take the sprocket off. Drill a couple holes in the seal on each side and use sheetrock screws or a similar screw. Just drill a small hole, just big enough to get the screws started and turn them in a few turns, enough to hold good. Grip them with ViseGrips and pull the seal out. If needed, you can drill more than two holes. The pick set, like pictured above, can also be used to help get the seal out.
Put a little grease on the lip of the new seal and slide it over the shaft and make sure to start it straight. You will have to use a deep well socket or something long enough to go over the shaft to tap the seal in with. Set the socket on the seal and gently tap with a hammer and make sure you keep it straight or it can bend or leak. The outside of the socket should be about the same size as the outside of the seal. If you don't have a socket that will work, a thick piece of pipe will work, but make sure the end of the pipe isn't sharp or it might cut the seal.
Most 4-wheeler problems are caused by a loose nut connecting the handlebars and the seat!!
You only need two tools in life -- WD-40 and duct tape. If it doesn't move and should, use the WD-40. If it shouldn't move and does, use the duct tape.