I would definitely weld it. Or better yet replace the head. Aluminum has a crystalline structure, and once a crack starts, it doesn't stop (kind of like a crack in a windshield) You can weld both the outside and inside, and it will still be cracked in the middle. Vibration and heat will eventually cause the crack to come back. Heat causes the aluminum to expand, then it contracts when it cools. Every expansion/contraction cycle will make the crack worse. That's why I make such a big deal about warming up an aluminum engine as slowly as possible.
2016 Rebel, 2013 Royal Enfield B5, 2009 Genuine Stella, 2006 H-D Sportster 1200, 2006 Vespa GT200, 2001 Yamaha XT225
"I refuse to believe in a risk free society where the thrill of living is traded for the safety of existence." — Nick Ienatsch