The problem is these old systems run directly off the magneto generator coil which inherently produces an AC charge. whereas LED's are diodes and inherently poor in alternating current. That means without a rectifier to convert AC to DC, and usually also a battery to smooth the current, LED's tend to burn out quickly.
also watts are essentially power usage. since LED's are more efficient they make more light at lower wattage. that means in an unregulated system it can be easier to overload them also, and they burn out.
If you can find a similar watt LED it might work for a while, if you want to dig into doing it right with a rectifier regulator and charging system that's a good option, or for simplest direct replacement, get the stock replacements.