TYMTIT, where are you based?
I am a metal fabricator that does aluminium welding on the northside of brisbane, depending on the case base material (which with Yamaha is usually good) could tig weld up, the devcon is also a great idea but welding will stop further cracking, Vandy is spot on with the issue of ease to get at and the welding may be the cheap thing depanding on how easy to get at and alike (would need to see or see a picture) would set you back about 40 bucks tops for welding but a bit less for forum members
Send me a PM if you need a hand preferrably with a picture or you could drop in and see me (if on the Northside if on the southside try Vandy's man).
Shane