I'm not sure there is a best replacement plug, from what I've gathered there are two non-resistor options of choice, neither one of which is widely available:

Beru 14 F-7 DUO (K18) or 14 F-9 DUO (K80) - not sure which heat range is the right one, Beru's worldwide website lists the F-9. Available from www.kingsborne.com.

NGK BP5EFS. I've only been able to find these from an eBay retailer in the UK:

http://cgi.ebay.com/NGK-spark-plug-BP5EFS-plugs-BP5EF-S-check4-application_W0QQitemZ250086662247QQihZ015QQcategoryZ72205QQrdZ1QQssPageNameZWD2VQQcmdZViewItem

Shipping is fast, only took a couple days for me to get them in California. I haven't had the chance to install them yet though.

Anthony