the cdi wouldnt cause piston slop, thats like rings or maybe bearings in the bottom end, the cdi does tell it to fire but the stator and flywheel would be the one if your timing was off, if the cdi was bad it wouldnt spark at all, maybee you sheered off the woodruff key, its on the timing shaft, it holds the flywheel in timing. or it could be the ignition coil too, i had one that wouldnt idle, the coil was stuck on go and it would instantly rev to the max. i think its in the stator or magneto, your piston slap, or slop, is probably a totaly dif. problem, but here in MI you can get a fresh bore, ported and polished, and a new weisco piston and rings and bearing all for $210 i dont know cost in you area but it cant be too bad, just a new piston and bore, (rings and bearing included) for 150-160$ i have a new cdi ill sell you if you still think its that after checking all the other stuff, if you have a repair manual, and an ohm meter, your set, just test all the leads as it says in the book, if you dont have one, go to the diy section and look for on that says the blaster REAL shop manual. and download it, find yourself a digital ohm meter, i picked one up for 25$ at the local electrical supply store. you cant do it with an analog meter because the scale doesnt drop down i n the single digits. if you have any questions feel free to private message me. i just got mine all figured out with the electrical, so its all fresh in my head and its really not hard at all if you got the stuff. i may have other parts you want also, so let me know if your looking, i need a new set of rear tires 18 inchers, gatta fit blaster rims. as long as they hold air and have rubber send pics.
