That one is too old for any manuals I have. However I see there is someone selling spark plugs for it on ebay, and he says the correct plug is an NGK B6HS:

http://www.ebay.com.au/itm/NGK-Spar...es-G20-G30-G35-G40-G41-G42-/140762970900

You might try cross-referencing this in a spark plug recommendations manual in your local speedshop.