As you are in France I'd rather think about getting a 100mm fork with disc brakes from Ben Cooper / Kinectics than to spend serious money for a drum brake on the front of the Brompton if breaking power in the wet and rim wear ist the problem that you want to solve. How many kms did you do on your Brommi in these two years?

Spreading the 74mm fork to 100mm does not sound like a good idea to me. Also in terms of electric support the fork ususally has to be spread to ~80mm on the Brompton, not to the 100mm of normal forks as this is what most motors that are used on the Brompton require.