I've finished the campaign mode but I missed some missions (the ones of the crystal and others) and some armory and lab upgrades.
Is there any way I can continue the campaign and finishing those missions and researchs? Is starting a new campaing the only way to do so?
If you press "Continue Campaign" after finishing the last mission, it will bring up a window similar to the Missions Archives you could access during the game. For all of the missions where you could choose one mission or the other, both options will be available to play here.
The only mission that you can't play if you missed it is the Secret Mission. To do that, you'll either need to start a new campaign or load a save game from before you finished the mission where you unlock the Secret Mission.
If you choose "load" (from the single-player menu in the StarCraft game, or from the F10 menu in the campaign) you should have several auto-saved options to choose from. This won't start you over, but start you back at places where you have saved, or where the game auto-saved.
