[C320-list] Impeller cover damage

[Martin Raphael]

As I was getting my engined warmed up this weekend I noticed water on the 
sole and realized that the cover over the impeller was missing a screw and 
thus salt water was leaking from behind the cover -- I then found that 2 
more screws had corroded and the heads broke off as I was adjusting them.  I 
am looking for advice on how to remove the broken screws from the housing --  
will I need to drill them out and retap for larger screws?  My engine is a 
1994 Perkins M30.  Thanks for any advice you can offer.
Martin Raphael
Rhapsody #108