It'd be cheaper and easier to swap the engine over.
If you want to stick with a 1.4, you could get a 1.4 16v engine from a Polo, which has 100bhp. Or you could get a 1.6 16v from the Polo or Lupo GTI.
Getting a 1.4 16v from a crashed polo would probably cost a few hundred for the engine, then a few hundred more for the fitting, meaning the bill would be at least a grand, and would that really be worthwhile. Once the mods are declared to the insurance it'd be no cheaper than insuring a GTI, and you could just buy a golf GTI for a grand anyway.