Universal - the manufacturer of the engine will mail you a free ring if you send them the serial number of the engine. They keep track of which engines have requested the ring.
If they are no longer offering this ring - please contact me and I will ship you a spare one that I have. It goes on M35B engine.
I have a write up here  http://mei1970.org/SailLaVie/OberdorferReplacement/HomePage-FrameSet.htm . Download and veiw bulleting 235 

Here is the bulletin below:

On August of 2002 Westerbeke issued service bulletin 235 regarding failure of sea water drive tang on cam shaft on early "B" model Universal engines.

Westerbeke will ship you a new tang re-enforcement sleeve, that is installed on the engine side, free of charge along with gasget for the pump.
Contact Westerbeke by phone 508 823-7677 or email help@westerbeke.com
You must provide your engine model and serial number. For me not only they send the re-enforcement ring - they also send newly edited manuals for the engine all free of any charge.
Here is the PDF file for the Service Bulletin 235
The part number is 48245
 

Steve-
You can search this site for info on cam shaft failures. It seems that there are maybe a handfull of owners that have reported the problem. I don't know how long the M35 has been used on the C36, but I would guess it's close to a thousand boats? 5-6 reported failures divided by a thousand is a pretty small number. Even doubled or tripled, it's still a small failure rate.