You will need to remove the flywheel. Use a flywheel puller to remove the flywheel. Mark the stator plate for timing if it isnt already marked. Remove the kick start assembly. Next remove the transmission cover and the clutch. Then slide out all the transmission parts (the kick start shaft and gear cluster are chained together so they will need to come out at the same time. Pay attention to the shims that go on the ends of the shafts. The main gear and sprocket can stay attached to the left case (unless you want to replace the seal behind the sprocket while you are at it. Now remove all the allen head bolts that hold the cases together. Thread the flywheel and clutch nuts back on the end of the crankshaft until they are flush with the end. Now holding the case in one hand tap the end of the crankshaft squarely with a rubber or plastic hammer. It should take much force for them to start popping apart. Get one side off, and then do the other. Remove the flywheel and clutch nuts and remove the case halves from the crankshaft. Pay attention to the dowel pin locations and sit them aside somewhere. The Crank bearings usually stay in the cases. At this point you probably want to replace the main bearings and crank seals. Pick the crank seals out. Now heat the cases with a propane torch until they are fairly warm and smack them inside down on a piece of wood like a 2x6. The bearings should drop right out. Since you cant replace it easily you are going to want to flush out the large end connecting rod bearing with carburetor cleaner or something like that to remove all shavings. Once the crankshaft is completely spotless coat it in some motor oil and put it in a plastic bag. Once everything is clean and ready to go back together you basically reverse the process. The crank bearings should go right on the crank. But you can heat the bearings in the oven for a short time if needed. Now with the bearings on the crank you need to get the cases around the crank. Heat the cases with the propane torch around the bearing area until they are nice and toasty. Install the dowel pins and the new center gasket. Tap the cases together with the rubber hammer, making sure the dowel pins line up and the case gasket stays in place. The bearing need to seat completely in the case or the crank will bind. Now that the cases are together put all the allen screws back in. The crank should spin freely! Now clean the cases where the seals fit. Oil the center of the seal but not the outside part, and carefully press it in until it fits flush with the case. Repeat on the other side. Now slide the transmission back in, and install the clutch. Install the transmission end shims, new transmission gasket and put the transmission cover back on. Now install a new base gasket. If your installing a new kit now would be the time to gap the rings. Install the piston/cylinder/head. Now put the kick-start lever back on. Last is the ignition.
Install the stator at stock specs. Install the flywheel. Fill the transmission and start the engine up! Now you can set the timing. http://www.mopedarmy.com/wiki/Ignition_timing_for_CDI
That's just a quick guide and i might have skipped something. If you need more info on installing the bearings, getting the cases back together etc, I'm sure somebody will chime in if you ask.