The LD spark plugs that come with most kits, while adequate to get a motorized bike functioning, may not have the most consistent sparking. In this video is a demonstration between a normal NGK spark plug and an NGK iridium spark plug.
As you can see, the iridium spark plug has more consistent and even arcing than the normal plug. Replacing the stock LD spark plug with an NGK spark plug would already increase engine performance, an iridium NGK spark plug could mean even greater performance!
Iridium plugs can be purchased at sickbikeparts.com. Remember however, that when replacing the stock spark plug with any kind of spark plug, to make sure that the plug is not going too far into the cylinder(which may cause it to collide with the piston). Simply remove the head of your engine and compare it to how the stock spark plug fits. If you think it goes too far into the cylinder, add some washers to the plug to fit.
A warning to those looking to mod their motorized bicycles:
Most, if not all, of the hacks and mods featured on this site were made by individual hobbyists. If you are new to working with 2-stroke bicycle engines, please by aware that there's a possibility that a mod featured on this site could seriously damage your engine. Please don't try any mod from this or any other site unless you fully understand what you are doing. The owner of this blog will not be held responsible for both material and bodily damage caused by performing a modification featured on this blog. Also remember that opening up your engine may void your warranty!