The advantage of a magneto ignition is simplicity. Everything is contained in one unit, with no need for a seperate electrical system. In the early days of VW aeroengines, a lot of agricultural and industrial engines used magnetos, so the early conversions, including the plans sold to this day, used them. Nowadays, it is virtually unheard of for commonly used engines to use seperate magnetos, and they are consequently rather scarce. Plus, they were often quite heavy. They were typically driven from the flywheel end of the engine, via gears or chains, or sometimes a direct coupling. The Ardem, Peacock, RSA and Falconar conversions are typical of this type.
Early US conversions used this type of mag, but they also used various brands of magnetos that plugged into the existing distributor hole. Vertex still makes this type of mag. This can lead to a rather awkward looking installation, as the magneto is taller, wider and heavier than the stock distrubutor, and leads to a "beauty bump" in the cowling of an enclosed engine.
Revmaster seems to have been the first to market an engine with a true aircraft magneto, initially a Bendix unit with two magnetos from a single drive, which was mounted to the flywheel end of the crank via a seperate accessory case which also included a permanent magnet alternator. Competing accessory cases from Diehl and HAPI followed this pattern, but used smaller and lighter Slick single magnetos, and this has become the most prevalent form of magneto ignition. Great Plains makes a very light magneto mount without an accessory case or alternator that is a fine setup for the lightest, simplest, hand-prop applications.
Magneto ignitions are relatively trouble free, are familar to most pilots and mechanics, but require the use of shielded aircraft plugs (or special adapters), and the harnesses and the mags themselves are relatively expensive. The spark energy also tends to be a little low, and rather narrow gaps must thus be used, which increases the possibilty of fouling when compared to other systems.
Stock Distributor/Coil Edit
The advent of the accessory case/alternator made attractive a cheap solution to the ignition problem: use the stock system. A Bosch 009 distributor, with purely centrifugal advance (other VW distributors have vacuum advance/retard, and are less suitable), and a "Blue Coil" were fitted to a great many of the early Monnett conversions - John Monnett was a great advocate of this system, and Steve Bennet of Great Plains likes it a lot as well. A cap with the leads exiting 90 degrees from the axis of the distributor is available to make a somewhat cleaner installation. This is still a viable plan, particularly with modern points replacements. This and all the following systems require some sort of electrical system in addition to the ignition components.
Stock Distributor/CDI Edit
Taking this plan on step further, using a transistor controlled capacitor instead of the properties of the coils inductance to discharge the ignition, this scheme provides a hotter spark, at the expense of some cost, complexity, and current draw.
Distributorless Electronic Ignitions Edit
The latest evolution, usually a "Waste Spark" systems that discharge two cylinders at a time. The Compufire DIS-X is the most popular VW system. The Leburg ignition is specially designed for dual ignition aviation applications. Revmaster now uses this type of system, as does AeroVee. All solid state, multiple coils, they provide a certain degree of redundancy, high reliability, and low maintenance.