Definatly try the bulb first, cheap and easy.
However, don't be suprised if it ends up being a failed rotor. The speed of a boxer crank cycles up and down every revolution, and with the direct coupled alternator, that gives the windings a pretty good shaking, thus airhead alternator rotors are a fairly high failure item. The good news is that rebuilds are readilly available, and halfway reasonably priced...economy of scale.
Unless your rich, don't just try things, it's not that hard to pinpoint the cause before shelling out for parts.
Sincerity is the most important thing. Once you learn to fake that, the rest is easy.