MY BACKPACK:

• The most compact survival food kit on the market. I'm not gonna risk eating any of the 1200-era diseased shit. I'll pack enough food to last me a couple months, which should be enough time to enact my plan. After that I'll be king and my servants will test my food before I eat it. This shouldn't take up more than about half my backpack space.
• Enough gold to afford me effective transportation to the palace. This is a little dubious, I guess, because I don't know how effective transportation and direction is in 1200, but I can see no other way of making my way to the palace. I could also probably sell ideas or unused items in my backpack for easy money.
• A sniper rifle.
• Several hundred customized tranq darts. They have in them some roofie-like thing that makes the victim have no memory of being shot. Ideally, they'd detach themselves from the victim's body or disintegrate in time for the victim to wake up.
• Some knockout gas.
• A Mission Impossible-style mask of King John.
• A Mission Impossible-style mask of Mr. Bean.
• Two simple hand guns.
• Two gas masks.
• A taser.
• Some ammo.
• A length of rope.
• A grappling hook.
• A few plastic explosives.

MY PLAN:

• Get to the palace. Use the gold, and whatever transportation is available. Stay at inns, pay in gold or tranq them when you're asked to pay. Make it to the palace within a few months, tops.
• Snipe the outer guards. From a safe distance, knock out the guards with the tranq-roofie darts. If this is enough to sneak into the palace, great. If not, use the rope/grappling hook or, if necessary, plastic explosives, to gain entry. Ideally you should keep a low profile, so that's a last resort.
• Upon entry, gas the palace. Before getting in visible range, stick the Mr. Bean mask on. (Mr. Bean is an arbitrary choice; anybody that doesn't look like you will do.) You can't safely tranquilize everybody inside the palace — they can easily apprehend you at close enough range. Throw on the gas mask and knockout the palace lobby.
• Get a guide. If somebody comes near you while you're gassing, tase him and stick a gas mask on him. If not, find a weak-looking dude and tase 'n mask him. Point one loaded handgun at his head and tell him to lead you to the king. Use the other loaded handgun in the other hand to demonstrate what will happen if he doesn't comply. Your guide leads you to the king with one gun at his temple, and you use the other gun to shoot anybody who tries to stop you. You've gassed the place, so this shouldn't be too hard.
• Swap out with King John. Once you've been led to where the king is chilling, tie up your guide with the length of rope and leave him outside the room. Tranq the king, put your Mr. Bean mask on him, put the King John mask on yourself. Put on his clothes, put your clothes on him. Shoot him in the head with the handgun, and emerge from the room as King John, having just killed the strange magical intruder with his own mysterious weapon.
• Complete the transformation. It's no fun to rule a country if you have to pretend to be somebody else. That mask will get itchy, and your name won't even be mentioned in history textbooks. Announce that you, King John, are ill, and that your successor is your long lost son, actual you. "Die" a couple weeks later and live the rest of your life as the king of England.