Very difficult. The conversion requires a great deal of expertise or a great deal of money or some linear combination of the two.
You can buy cards in a few places (
www.eio.com comes to mind), but they're not that cheap and they are adapted to a very specific lcd screen. That's where the rub comes in. There aren't really any universal standards for laptop displays. You have to exactly match the specs of the card with the LCD.
It can be done and there are much more technically inclined folks out there that can help. Given the drop in LCD prices in the last couple of years, however, it just doesn't make sense to invest the money and time in the conversion, unless you've got some exceptional reason. If it's for size, go check out some of the auctions on ebay for surplus industrial screens that DO connect to analog VGA.
Good luck!