Hi Chrish,
You will find it hard to get any positive info about the correct grade of spark plug for your bike because it just predates such information. People were just starting to realise there was a lot more to sparkplugs than anyone realised. It wasn't until the late 1920's and 30's that sparkplug manufacturers started to actually started to put out a range of plugs for different conditions and different engines. Even then these ranges weren't great compared to today. Any modern sparkplug is miles better than the plugs sold when your bike was new even if it isn't "correct".
Chaterlea25 has recommended a good firm to buy a spare plug. They can provide something appropriate but basically, you don't have anything to worry about, you can't hurt your engine no matter what you put in it. If the grade is much too soft, the plug mightn't last more than a few years. If much too hard it might (might) oil up. Neither will hurt your engine, you will just be inconvenienced by the side of the road while you change the plug and how long does that take on a Round tank?
Happy riding,
