Thelonius,
OK. Here some information you can look at:
Here you will find information on performance
http://www.centuryperformance.com/spark.asp , then you can understand letters and numbers on this site that have explanation for mayor brands
http://www.sparkplugs.co.uk/pages/te..._technical.htm , and finally here you will fine what you are looking for, it is very easy to get even for your grandpa bultaco
http://www.sparkplugs.com/default.asp?KID=2518 , here you will be able to look plugs for any application you want.
quick answer are these:
EC 250, I guess is the one you have, 1997-2003 NGK B8EGV, 2004-2005 NGK BR8EIX, BR8EG.
Cheers,
Gus