Chain adjuster broke

[DANVAN]

My buddy has a 2002 300ex, the chain was very loose, so we attempted to adjust it, the adjuster seemed to be all the way at the end, but the chain was still loose, so with all the presure on it, I broke the aluminum casing which is the adjuster. So now besides the chain still being loose, Im wondering how do we change this adjuster, and if we dont chage it, will dirt and mud get into the rearend housing and damage bearings?
Thanks for any info on this.