I'm afraid that there's no simple answer to this question. If you do go for an upgraded engine, you'll have to fork out for a new unit or buy a used one, strip it and make sure it's okay. Then you'll have the associated costs of fitting it and all the little modifications that are required. And you might not even be adding value to your car as it'll no longer be original. How handy are you with the spanners? I think the best thing to do would be to get your engine out and take it apart and find out exactly what's wrong with it and what caused it. These engines are fairly simple and parts are plentiful.