I have an M1A NM and a Winchester M14. Both are very accurate and fun to shoot, but a bit on the heavy side to tote all day long. I also have an FAL (Imbel) and it's a lot of fun to shoot too. The biggest drawback to the FAL is that the barrel gets pretty hot if you fire more than 1-2 rounds a minute thru it. But it balances very well and is easy to aim and fire, pretty natural to aim.
I don't care about customizing either style gun. They are extremely well made and other than optics I can't think of anything I'd add to either. Nothing they really need. How do you improve on perfection??