A Core 2 Duo T5750 may be the best match to your socket, bus speed, voltage, and power requirements. Even the clock speed and multiplier match up, but it's dual core with a nice doubling of L2 cache to 2MB.
These are not expensive, $10-$15 fixed price on eBay. Would be worth that and a $5 syringe of thermal paste just to try it out. If it doesn't work, you're only out $20 and you can resell the T5750 for half that or so, right?
Of course a new $350 laptop with a Pentium B9x0 would be VERY cheap, and give you plenty more power, while a $450ish laptop with the Core i3-21x0 would be even snappier. You might find a Core i5 Sandy Bridge laptop close to $500 if you look hard enough.
I have upgraded the single core Celeron 900 in my Acer 5230E to a Pentium T4300 dual core chip and it's been awesome. With the recent frying of my Dell system due to voltage loss in a brownout, I have been using it daily as a desktop replacement. I know it's slower than my quad core Dell was, but it's keeping up with me for work purposes just fine. I am toying with the idea of a T8100/T8300 CPU in this same laptop just to gain more L2 cache and see if I can notice the different. Little $10-$20 upgrades can be worth much more in added productivity than changing everything and spending $300-500 on a new system.
I encourage you to try a T5750 (or the slower T5550 or T5450 if the T5750's sell out and the slower ones are found more easily) and see how it does. Just having the second core will make all the difference if you even choose the slowest Core 2 Duo, the T5250, or one of the Pentium models in 667MHz FSB, the T3200 or T3400, or one of the Celeron dual-core models in 667MHz FSB, the T1600 or T1700. Any of these are WELL under $20 on eBay and worth a shot.
edit: I see these Proline laptops are sold in India and South Africa and it might not be possible for you to buy on eBay, but I'm sure these chips are not expensive where you are since they are four to six years old.