Most cars build after the 1970s seem to have very few problems with unleaded fuel. As long at the proper octane leve for your engine is used, the ignition won't need any modifications. As you will probably understand by now, there is always some risc involved. There are some good additives available. Red-line and Wynns produce good proven additives.
Some additives also provide an octane boost.
Most additives need to be mixed into the fuel properly and will sink tot he bottom if the car is left standing for a longer period of time. Therefore it is better to store the car with a full tank of unleaded petrol if it will not run for more then a month. Convert the engine to unleaded. This involves replacing the exhaust valves and seats by hardened ones as used in modern engines. Me and my friends use original unconverted 602cc and 652cc engines with unleaded 98 octane fuel.