I currently have a chip about the size of a silver dollar on the lower gear case (Alpha One) right on the fin at the point were the lower housing meets the upper.

My question is to what options would I have to repair this and make it look as unnoticeable as possible?

Obviously a new lower gear case would be best but is not cheap.

Weld (TIG) a matching piece in place but would take grinding the weld and possible filler and paint to be unnoticeable. I know that the paint will probably not hold for very long.

The gear case does not have a hole in it just on the external fin.

Any suggestions??

try JB welding a piece in. stuff is pretty tough. i hit a curb with my car automatic trans. knocked out a hunk about same size. all the tranny fluid fell out. found the chunk, cleaned it up with acetone and JB'd it back in. 6 months and counting...you can also lay a larger piece underneath and glue-clamp it on. then bodywork it out. (i think they call it fairing when it is marine, sounds better than bondoing.)