You could kill the dragon, but since it's been done before you'll first need to craft 4 end crystals in order to respawn it. That's vanilla gameplay so no worries. The only differences are that it won't drop nearly as much XP as the first time it was killed and it won't drop the dragon egg (there's only one of those per world and on this server it's already on display in the spawn area).

The 2 biggest mods in place are teleports, which you don't need to use, and land claims, which you also don't need to use but it's useful for protecting your stuff from griefers and thieves.

Also Minecraft is a sandbox game, so feel free to use or not use any of the mods or preexisting builds, portals, etc to get the job done. Play the game however you want to play it.