I'd suggest you go to a local marine diesel mechanic. The elbows, sometimes called mixing elbows, are made of cast iron and have a usable life of 6-8 years. They mix raw water with exhaust gases for cooling.
I just replaced the elbow on my Olson 911se. Disconnect the raw water intake hose (close the thru-hull first) and the hose that goes into your exhaust riser. Remove the elbow and flange from the rear of your engine and take the whole thing to your mechanic and ask for a replacement. He should remove the elbow from the flange that attached to the engine and install the new elbow. Make sure you get a replacement gasket and gasket dope from him. To avoid leaks when reassembling, make sure you clean the flange face (the part that mates to the engine) and the matching surface on the engine.
I'm not mechanical by any stretch and did the procedure (out and in) in half a day. A mechanical person could probably do it in an hour or less. Good luck.