At the very least make sure you've cleaned the inside of the casing. Make sure that the gasket sealing surfaces are clean.
Remove clean and replace the small parts that fit in the casing
If you are replacing the output shaft bearing; fit the new one in place by tapping on the outer part of the bearing race with a copper punch. When the bearing is seated fit the circlip. Fit the output shaft seal.
Refit the speedometer drive, the secondary pump cam, and the governor.
Fit the secondary pump piston and make sure you align the hole for the securing bolt correctly.
Do not fit the secondary pump spring and cap just yet.