It will take the full event to fully upgrade your buildings, unless you use diamonds to speed up the process. You will earn upgrades by scoring goals, finishing certain amounts of quests, and I believe in the tournaments.
Previous event buildings...Older event building upgrades can be found in the antique shop and also may appear during events. Recent event buildings, you will have to wait till next years event of the same season to see again. Like the Airship will not show up again until next years archeology event.