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Great, that makes it easy.
You'll find a mix of 1 watt and 3 watt conversions - I think you'll need a 3 watt to match or exceed the original Mag LED assembly. There are some 5-watt conversions offered but they tend to need 6V or higher, ie more than you will get from 3 D cells.
This form factor seems to be one of the better options for 3W - worked fine on my 2D and 4D hosts but would not thread onto a 2C. Seems to have better heatsinking and so higher continuous output than the run-of-the-mill 3W options but not a huge increase:
325 lumen instant upgrade with Luminus SST40 LED for super bright light and long range.
lumencraft.com
Once you get into the 10W and higher range they become an option for a 3D light again because they tend to have more electronics in the assembly. I have one of these in my 4D host - it's pretty bright and has 3 modes if you want a do-everything light:
Shop now. High power LED upgrade for 3-6D cell Maglites. Multi level brightness.
lumencraft.com