make sure all the seals around area are out the ali gets super hot , use clutch brake cleaner to clean out the crack or youl get shed loads of porosity,casings are crap to weld done my mates yz125 ..it was a struggle, i personaly would set up on a scrap piece of ali first ,ill try and find out some settings 2moz if thats any help ( i work for ozweld international).
dont forget to drill the end of the crack, stops it creeping...if its possible