If you go above a 143 cc kit you have to deal with issues that would be complex ,such as dealing with the proper AFR ,requiring a fuel controller? or a temp gauge ,with monitoring the spark plug while adding either two cycle, oil to gas or marvel mystery oil at a ratio that would vary depending on different conditions ,which I believe would probably work, but it would require experimentation with regard to actual application, (my idea) ( which is why I can't explain or recommend) varying conditions , etc, negating the ability to apply simple math in equations ,to reach a simple solution regarding the application, ( engineering ). How hot is it where you are operating? air flow rate over the engine, when will the effect of the additional heat added by larger displacement no longer be removed resulting in component failure. The ( sno ball ) effect ) ( critical mass ) ? The addition of heat increases resistance, resulting in an increase of heat ? Bigger cylinder,smaller block heats up faster, resulting in the inability to remove additional heat added in an air cooled engine. Keep it as simple as you can. I believe I have achieved the full practical potential of the bike without compromising reliability,with the frame it rolls on. I am happy as a clam with it now! It is FUN to ride, it sucked when I got it. I do not believe the engineers either ever rode it , or were not a factor in what was actually marketed. Also I went back to the 13 tooth front sprocket, big difference! It doesn't affect the top end speed,it will allow you to achieve it on hills.