The reason that modern LCD and plasma televisions are great for just about everything else is the same reason that they can’t play old, 8-bit games for s***.
Sadly, there is no hack or workaround for this. The best advice I can give you is to go buy an old-school cathode ray tube the size of your LCD. That would do the trick and only take up a third of your living room in the process.
The problem, put simply, is that modern televisions read and display the pixels in a way that doesn’t allow for them to be resized proportionally. Ironically, the old televisions can do it just fine.
Before you get too worked up trying to figure out how to play Duck Hunt in stunning HD, though, consider what the author of the article has to say:
Trying too hard to replicate the failings of old hardware is probably a sign of mental illness, but we do it anyway. I can't abide the idea that it's better or that the games were designed that way, or even that the designers intended them to be played with scanlines... It's just a silly pursuit which we engage in because we are chasing a personal ideal, a fleeting and enjoyable memory.
Source: Game Set Watch