Here are three sources we have been having good luck with for M50 parts:

TAD Diesel  -  They are in Virginia http://www.tadiesels.com/

Stewart's Marine -  206-789-4600 or 800-735-7207 he is in Seattle and has some good used parts.

Pacific Power -  360-887-7400  Ridgefield Washington. They are a Perkins dealer.

Our parts book shows P/N 2848A227 for the starter relay. We have had some starter trouble and were looking for the relay. I think we ended up finding it by the numbers that were printed on it. May have been a Lucas 33ra but we never replaced it because we found the problem was the pushbutton switches in the panel one time and a blown engine ground fuse another time, so we have not replaced the relay on ours.

Good luck,

John