Usually the bulbs indicate their wattage on the side of the metal base.
You might try ordering one from moped junkyard or moped warehouse that you know is the right wattage for your bike.
You can find the correct wattage from the users manual, which can be downloaded online.
I had the same problem with one of my bikes--the tail like was really dim. It turned out that it was an 8 watt bulb when it should have been a 5 watt bulb. When I put the right bulb in it was much brighter.