The most commonly recommended spark plug for the Isetta 300 is the Bosch W7AC (WR7AC new version). It used to be the Bosch W4AC, however many people feel the W4AC does not work as well as the W7AC with modern fuels. Speaking of modern fuels, if you live in the US or Canada and you're concerned about using ethanol blended fuels in your vintage vehicles, there's a website called pure-gas.org that list gas stations by state that sell ethanol free gas. Of course there are many other brands of spark plugs that work in Isetta's. Another popular choice is the NKG B6HS. People have also reported good results using plugs with a longer reach (thread) such as the Champion N6YC. If you're wanting to use a brand other than Bosch, NGK, or Champion, a good resource for cross referencing spark plug brands is Club Plug USA. Another useful online resource for diagnosing engine/spark plug related problems is NGK's "How To Read A Spark Plug"
The spark plug recommended in the Isetta 1957 Export Owners Manual and Repair Manual is the Bosch W240T1. These NOS W240T1's recently popped up on ebay. They're not cheap, but if you're planning on entering your Isetta in the Pebble Beach Concours d'Elegance these babies will probably score you some extra points. If you're planning on actually driving your Isetta, I'd go with some much less expensive and better performing Bosch W7AC's.
An old Lodge spark plug cross reference chart. It shows the Bosch W240T1 being interchangeable with the Lodge HH14, Champion LA10, and KLG F80.