Since no one has replied to your question, I'll take a stab at it. Let me first say that I have absolutely no experience repairing IB/OB drives by welding. I just have a lot of "Book knowledge" from extensive reading on various topics.

Since your new welding business may require you to identify the type of aluminum alloy (or at least its alloy class) a broken part was made of before you attempt to repair it, you might consider investing in an aluminum alloy test kit, such as one like this:
http://www.walkerscientific.com/Walker/NDT/chemsortkits.html
This is one of several brands of test kits. Perhaps one of the pro welding experts who contribute to this forum might comment on the usefulness and accuracy of these kits.

Search "Welding skeg" with Google and you will find a large number of websites discussing repairing of broken off skegs and damaged drive housings. There are also some forums dedicated to Merc Drives. These sites may provide you with helpful information.

I have to weld skegs all the time at my work. Here is what I have discovered over the years. I use reg o'l 5356 filler. The metal is cast and the sand in the cast is the worst issue. Either way,they make replacement skegs of various thickness and shape.I have found that welding them with this rod is definately strong enough.Have seen some come back bent like the fin on the whale in "Free Willy" and the weld doesnt fail.I use a double bevel with a 1/16th gap so that I can get as much clean aluminum at the joint. Sometimes you have to weld it once,grind the impurities and pinholes and reweld. Also,if the person with the boat is a bottom driver,it may be better for the skeg to break away other than to crack the gear housing. Fixed alot of those too. One thing to consider when welding lower units,to avoid damaging the prop shaft seal,tilt the motor housing up so that you can fill the housing with water around the propshaft.