In addition to the previously mentioned MIG uphill, I'd like to get more familiar with, and better at, Dual Shield Flux Core. 

Same here, I suck at uphill.

Paul Charron YW. Whatever route you go, I'd first trace out a nice accurate template of the part. Then, if the weld option goes sideways, you'll have a plan B.

While it would be interesting to try and do a weld repair( after all, we are welders! ); unless there's a specific reason for it to be made of a casting, I'd make a new one out of plate. It's a simple looking part, and could be reproduced pretty quickly with just a drill press, bandsaw, and files. Would be less likely to break as well, as that piece that snapped off looks like a problem waiting to happen for a cast part.

Good stuff. Welcome Shaun!

If you have a milling machine( or have a friend who does ), the stubs of aluminum filler are great for a pad/pressure point between the moving jaw of the vise and the workpiece when initially squaring up stock. 

In addition to the internal pressure already mentioned, it could also be that there were some contaminants inside, and they pooled/migrated to the low spot and now getting sucked into the weld puddle from within?

Aren Jenkins It probably will be fine, but since it will be at 200F+ and be subjected to plenty of vibration( the motor is bolted directly to the frame, so the whole bike shakes ) It'll likely be a re-do. The tank sits above and in front of the rear tire, and even a slight oil leak at the wrong moment, could be a disaster. Should be able to do a much better job welding it this time as well. The old one can sit on a shelf with a note on it......"Do Your Research First!" 😀

Aaron Laughlin Not something that I could really answer. All I can say is that I find value in it. 

Aaron Laughlin I would hesitate to call myself a good welder. More like functional and somewhat competent. I just suggested checking the gas flow, as just the other day I forgot to turn the gas back on, lit up the torch, and experienced the same thing, and fried the diffuser in a Furick Jazzy 10 in the process! From what I've seen, all of the coaches/instructors on here bring a lot to the table, and from a really divers background. Maybe just browse the videos, and see whose specialty most closely aligns with what you want out of welding. Though I like watching them all, as you can learn things from people who work on stuff you may never even see. Always something to learn.