My 2000 Prostar 205 with the LTR motor had a similar failure. The bottom bolt that holds the alternator to the engine bracket had failed left a small 1" stud on the bracket, bolt must be somewhere in the bilge (I did not look for it). The stud eventually vibrated out and the alternator rotated inward (pivoting on the upper bolt) it then started to rub against the tensioner.
My quick fix was to just replace the bolt but I suspect it may need the update bracket as well. Does anyone know what grade bolts I should be using. The bolt that did not fail was marked as ASTM Grade 5.2 and that is what the new bolt is as well. I did not torque the new bolt (not sure I can get my torque wrench on it anyway) will torque the upper bolt before next use though. I assume that the bracket upgrade is needed anyway.