I do not like that mod at all and i knew it would give problems the moment i seen it but without looking at your device i would think that the usb transceiver chip has gone but i could be wrong, you could take it apart and look at it and associated components, too many to point at sorry to say, and if any have fried anything you will see straight away i am sure.

Double check all your wiring in case you have made any shorts with the pads etc and if you get it apart try to link wire the port back in place, not easy i know but hell it is worth a try, i only wish i had it here to look at.

After looking at the pic which is not good, i can see components missing and the board itself is not the same shape as the one actually inside of the N900 but hell it aint a lot of money so see what you end up with.

Sorry i can not help you further unless you go inside and check everything.