To reiterate what Proximon said earlier, you would need to find a spare/replacement laptop motherboard from a modern laptop that works with an i7. I couldn't find any with a quick search on ebay...it's likely you'd have to buy them direct from the manufacturer, if they're even willing to sell to you (which is unlikely). Then you would need to verify that it fits in your laptop case. Chances of that working are slim, laptops are designed to work with the parts they came with, and don't deal well with change themselves.
I'd say it's less of a question of money than a question of being able to get the parts. They're just not widely available.
You really are better off buying a new laptop, even though it involves change. The effort, time, money, and frustration that would go into trying to get your old one to work wouldn't be worth the effort.