NGK is the spark plug maker.
For the 48/66cc engines they have a standard version, and an Iridium version.
The only number in the long letter code part number is the Heat Range number.
The higher the number, the more heat the plug lets out to the head.
You want it as high as you can without fouling the plug.
The regular version is
NGK 5111 (BP7HS)
The Iridium is
NGK 5944 (BPR7HIX)
Both are longer than stock for a bit more compression.
Both are excellent plugs.
Iridium's are twice as much but will last the engines lifetime and come pre-gapped for best performance.
http://SickBikeParts.com carries them all and where I get mine though you can order them direct from
http://NGK.com buy the short 4 digit number.