I spent a while thinking about doing something similar. I ended up buying a new headlight, partially because I didn't have a suitable housing or old light body to convert. DIY optics didn't appeal! As you do already have something suitable, you could fit something like this:

XMLAWT-00-0000-0000T5050-STAR - MULTICOMP - LED Module, Chip on Board, XLamp Series, Board + LED, Cool White, 8300 K, 260 lm | Newark element14

It has a maximum current of 700mA, but you may not need all of that as it will be very bright. 700mA * 2.9V (the LED voltage drop) = 2W max. power consumption.

(This is only an example, there are plenty of other similar options that may be more suitable).

At the very least, you'll need to use a series resistor to limit the current. It may work if you directly connect it to the dynamo output, but not for very long.

Adding a bridge rectifier will double the power going to the LED, and will also pulse less. I once made up a little circuit that uses a bridge rectifier and a transistor to regulate power to the LED - it uses 8 components, and worked really well (although some of the components need proper sizing for power dissipation, or they will melt )

If you are thinking of using the internals from a flashlight with a dynamo, you will still need a rectifier/adapter circuit.