Sea-Way Marine 1-800-332-0803
They specialize in older motors.
If all else fails, the crack can be fixed. I have a 60's model Evinrude that was given to me. It had cracked half way around the lower unit ( bad seals/water froze ) . I took the lower unit off, cleaned it thoroughly with acetone and a brass wire brush. A friend welded the outside of the lower unit, then I used J.B. Weld on the inside of the unit. I sanded the J.B.Weld smooth, cleaned it again and put it back together with new seals.
That was 8 years ago, still no leaks.
Oklahoma member since 2003