this thread made me watch a lets play of terraria so I could ponder it. As people have already said a 1:1 is not really going to happen but you could certainly do SOMETHING decent.
I'd probably start with a basic miner like miner dig deep (
[ Show youtube player ]
) which is a pretty darn fun game to start with. Then i'd just see how much I can add to it before things fall apart
You'd need to heavily optimize and make sure you are only dealing with things near the player I think - it means the rest of the world cant tick without you but hey, 7mhz.