I tried and succeed replacing the usb port today.

I de-soldered the old plug -which was broken inside- and replaced it with a new one that I ordered some years ago.
I had an old elecronics iron, an electrician one (bigger than electronics one), a good magnifier and a second new plug.
The hardest part was to remove the old plug. It was never hot enough, even with a big electrician iron. I had to be a bit...rough.

I cleaned the remaining pads (some, but nothing dramatic, parts of the pads stayed with the original plug) and the usb contacts and scratched the green enamel a bit larger around the plug place.

1. I carefully placed the new plug. Leave a tiny part of the pads visible to always be able to control if the plug didn't move. Once it was on the right position (regarding the usb legs), I melted FEW soldering material on one side without touching the plug, waiting the material to flow to the plug by capillarity. This way, the plug didn't move at all.
2. I soldered the other side
3. I came back on the first side to ensure it had been warm enough.

While doing 2. and 3., I pressed on the plug with a piece of wood to ensure it was as close as possible against the board.

DO NOT put too much solder material as it will migrate into the plug and fill it. (that's what happen to me on the first try)

Once these "mechanical" operations done, I did the "logical" part, the usb contact legs. Here, I didn't add any solder material but just used what was remaining on the board: error. Next time, i'll put some on the legs before to solder them.

It is a precise work and it should be done in calm, and with time.
Breath (not the soldering vapor though), dont hurry and keep confident! Order lots of plugs to admit the idea of failing the first time(s).

I tested charging and data transfer/reflashing, all is working.

Next is to see if it holds in the time.

It's a great pleasure to resuscitate an N900.
I have to say I woudn't stop and like keeping continuing these operations.

So members, if you want to throw some N900 with dead usb away, don't!
Repair them or send them to me!