What is the difference from a 1939 Indian Chief motor and a 1948?

I am working on a Indian Chief motorcycle rebuild/restore in my shop.

After researching the frame and motor numbers, I found I have a 1948 Frame with a 1939 Motor on it.

I am just trying to find out what the difference there is in the 1939 motor that's on it and a 1948 motor that should have been on it.

Thanks for any help.

JR
Briggs Bikes, Manhattan, KS