Hiccup had to work fast, but carefully. Through watching his dad gut animals, he learned that the heart actually contracts to pump blood. (The heart is a difficult muscle. Sometimes hours after death, it still pumps) He then got to work on designing an, artificial heart. Made out of other muscle tissues gathered from slaughtered livestock. It wasn't a real heart. A strike of lightning would cause it to come to life and contract as normal. Problem was getting lightning to hit it. So Hiccup put a metal coating around it. While Maverick was passed out, Thor was generous, and a lightning storm was about to hit. He removed Maverick's heart that was affected by Red Rabe. The heart was almost pitched black with evil.
Hiccup finished his prototype, which would also be the first one in use, and managed to get it into Maverick's chest. A seamstress then closed up the cut. Now the hard part, getting Maverick up to Gothi's hut. Gothi lived at the top of the mountain, any metal up there would surely get hit. But how so you get an unconscious five thousand pound Monstrous Nightmare up there?
Hiccup figured it out. Just like when Vitani was wounded, multiple dragons would grab him and lift him up there.
Hiccup: Just hang on Maverick, we're almost there. We'll get that thing working. I swear it.
They managed to get him up to the hut, and they instantly fled back down. They didn't want to be there when the lightning struck.
5 minutes later…
Hiccup: C'mon c'mon. Gives us some help Thor, Odin, or anyone.
Astrid: I don't think it's gonna happen. The storm is almost over.
Hiccup: Man this has been a short storm. Strike it, c'mon strike it. Just one strike.
crack… BOOOOOOOOOOM! BZZZZZZZZZT!
Maverick: OWOWOWOWOWOW SON OF A BITCH THAT HURTS!
Snotlout: Wow Hiccup, for once, your idea actually worked.
Astrid promptly punches Snotlout.
Astrid: Too soon, and not true.
Hiccup, Toothless, and Vitani flew up. Hiccup put his ear to Maverick's chest.
Tha-thump tha-thump tha-thump.
It's working just like it was designed.
'Hiccup I'm gonna do SUCH illegal things to you. Wait, I'm mad, but I'm not in Red Rage. What did you do?'
Vitani: That's not important right now. What is is that you're ok, and the Red Rage will no longer trouble you.
Most of the science here is actually true. Electricity DOES cause muscle, even dead ones, to move if there's enough of it. Which is where the idea of Frankenstein came from. The heart is just muscle tissue, so it probably isn't impossible to make a replica out of other muscle tissue. The heart does often beat a while after death. The only real fictitious thing is how long Maverick lasted without a heart. You can live for about dozen seconds without a heart until blood no longer reaching the brain causes it and you to die.