You want model 4782, not O600. The 4782 motor is fully enclosed, and I have one running beautifully on my 520.
Also, a 1/6 (even 1/4 and 1/3) HP motor will bolt on with no issues...but in my experience the 1/8 HP motors are usually the most quiet at startup and the least expensive. Even a 56N frame motor will fit...I have one on my brothers 520; it was new old stock and $50 to the door. Made in USA also.