For one you have a Skyhawk motor, you don't get a better motor anywhere no matter where you buy it. For two your elevation is going to make any motorized bike motor you get sing like a sick horse. Don't change the Gas to Fuel Ratio, keep it at 16-1 (8 oz of oil to 1 gallon of fuel) for now and after your break in switch to the 25-1 or 32-1 and with your elevation a 20-25 to 1 used at all times is better. The HP/Racing carb that you have is also a very nice Carb...But again if you are not interested in going over 40 MPH (which you really don't want to go this fast on a bike anyway) It don't matter if you use the HP or the regular NT, only difference is at top end. You don't want a cooler spark plug, you want a hotter one and yes the resister type is best (NGK BR6HS is perfect). I thought I also read that you were using 90 Octane fuel? No my friend use 92 octane and use good fuel (don't mess with the ARCO junk, I use Shell) Every 66cc 2 stroke should use 92 octane. I find that Kings motor bikes is the best vender (my personal opinion) its where I got my 7 motors from and there warranty is also 6 months and the costumer service is great. (very good vender/company) Now, the movement of the piston within the cylinder wall is unfortunate but instead of just tossing it and buying a new motor, first check your rings#1. if the movement is a back and forth movement of the piston it could be the rings, if its a side to side twisting your bearing is shot. and the person that told you its almost impossible to fix is incorrect its as easy as just changing the bearing (piston bearing) now if it went bad because of the sleeve (the rod) thats a indication for a new motor. (better to get a new motor then try to fix this problem, that would be a little difficult for the average person). Not to boast my position or anything like that but I do own 7 of these motorbikes and have been fixing and repairing 66cc 2 strokes for years. All the advise you have been given is very informative but you must (a big must) consider your elevation (it sounds like thats where most of your problem is coming from) High elevation make these machines run really weird. Believe me ,been there,done that to many.