Is the new motor connected for the correct voltage? If it wasn't ever connected from the factory(unusual), that would be the only way to get 500kohms line to line. Maybe there is an integral motor protection switch missing. These usually would be on the star or wye point of the 3 windings.
With an ohmeter on a motor that size what you are looking for is continuity, line to line, and none line to ground. The ohms are usually so low it is hard to diferentiate between normal and abnormal with a common ohmeter.
Jim