Hello Blue,Best performance tires are radial tires, but 18" rims limit your options. The selection of performance tires is far better in the 17" rim size and modern tires are typically much wider. Best rain tires I ever rode on are soft compound radial tires, the rubber compound and tire construction is what makes the difference way more then the tread pattern.
( Pirelli Diablo Rosso Corsa ) <- it's about a 300$ tire that you can't fit on that bike but wow do they ever stick good on the wet.
My two friends and I have been racing vintage bike for a few years. Most of the classes we are now competing in have rules that allow any wheel size but for you vintage racing is getting more and more popular all the time. Because of this new popularity the tire manufacturers are starting to produce race rubber tires in street (threaded) configurations because some of the race organizations do not allow slicks. If I remember correctly Metzler is coming out or has started building a line of tire that should work well on your motorbike. Race rubber will not get you the miles that street rubber would but they will do fairly well if maintained (keep checking the air pressure) and as TR said they are not bias ply old school tires but rather radials so they'll work much better.