I would go ahead and get it welded. The crack will grow and the damage will be worse. If you can find someone that can tig it from the inside the damage to the coating on the outside won't be very noticeable. You can probably touch it up with a little high temp aluminum paint. I've done that before with decent results. If that header has a slip joint on that tube that may be the reason it cracked. If that's the case you will want to add a bolt tab at the silp joint to keep it from cracking again.