What needs to be done to freshin up a Flat Head

That depends on how long it's been since it was done before, may need only rings and cylinder de-glazed, valve lapped, valve springs replaced if they are below the pressure you need at the installed height, carb cleaned and serviced with new pump. Possibly a rod insert or a new rod if it's been a real long time with that rod. Check the piston to cylinder clearance and bore condition to determine if you need to go up on bore size or need a new piston. That's pretty much it.