I need to weld a new skeg to my Bravo 1 X drive...Do I first have to dismantle the whole lower unit before welding or can I leave all the gears etc in it? No problem with the welding heat or distorsing afterwards?

I've often wondered that myself, But our local prop guy will fix our skegs right on the boat with no disassembly. Haven't had any problem, but I can imagine with to much heat anything could happen???!!

As long as there is at least 1" left of the old skeg it is not a problem. Aluminum is such a good conductor of heat that all that happens is the entire unit gets warm. The stress induced by the shrinkage of the weld is confined to the skeg. It helps to peen the weld as it is cooling. Weld an inch or so, beat the crap out of it with a hammer. Welds some more.

So Is I understand it would be better (if possible) only to replace only the damage part =(half of the skeg) instead of completely as furnirtured....

Yep, the less you replace the better, just trim as needed.

Where do you buy the skegs? I guess you can weld onto a good skeg to extend it longer.

I did this same thing last winter myself. Bought the skegs from BAM, which are actually just skegs cut off of new damaged drives, and used the needed section to repair mine. The weld line is visable about 1/2 up the skeg on the 1st pic. Once finish ground and repainted, can't be seen that mine were repaired. My other drive just needed the leading edge repaired. Same procedure, just use the area needed.