Don't know what to fix

So I've got this 20 inch iMac from a friend. He took it to the Apple Support, because after a lightning strike into the house it don't work anymore. They took out the harddrive, which was fine and all the data was fine as well. Then they said, they don't know whats wrong with the iMac, so he just bought a new one, because he wanted to buy a new one anyway.

So here is whats going on: When I start it, everything starts: Hard Drive, the Fans, the DVD Combo is checking if there is a disk, and even the Mac start up sounds comes, but nothing happens - the screen is staying black and the hard drive is turning on, but nothing is booting or something like this.

So I thought, may be its the display, so I used the mini DVI Adapter to use an external Display, but the Display doesn't show anything.

So, I thought, may be its the RAM. So I took out one RAM, but its still the same - tried the other one... same effect. And when I start it without any RAM, it's beeping (of course, this makes sense). So the mainboard is somehow getting, that there is no RAM, so it can't be completley dead, right?

But... in the end,... I think the mainboard is dead, what do you guys think? What could I try next???

And other question... if you think its dead, is it possible to get an adapter for the internal display, so I can but a raspi 2 inside the iMac???! Did you know if anyone ever tried it before?

Thanks, and greetings from Germany

Chris