Personally I wouldn't spend a cent on the existing motor. The old saying good money after bad. Over heating suggests the possibility of a lot of issues including cracked heads etc. I just wouldn't rebuild an old raw water cooled motor that has already been rebuilt.
Spend 7k on a new Beta and fit yourself with some help. If you have the money why worry that you would be spending more on the motor than boat is worth. If this means you can rely 100% on that motor for next 20 years it's gold.