In Europe/UK there have been numerous guys who have stuffed an Opel 6 cylinder engine into their Astra/Kadette.
The 2.5L V6 engines (C25XE/X25XE) will fit in and some people will do a 3.0L conversion where they use the bottom end from the C30XE with the top end of the C25XE.
The 3.0L V6 conversion with a few modified parts will make 245hp but it's heavier than the 2.0L 4cyl so it's a toss up as to which one is better.
Of course you could go crazy and do a 3.0L V6 turbo build up.... which would make some unreal power.... but that would cost a pile since it would involve a bunch of custom work (and I'm not sure how you would fit a turbo in there.... since the V6 swap makes things pretty tight as it is)
There are piles of Diesels available for the Astra/Kadette so you could do a conversion if your interested in saving some money by getting better mileage with cheaper fuel.
I'm not that familiar with the Diesel Astra's but I think the most common motor was a 1.7L Isuzu GM turbo diesel.
There are a lots of different diesels that would fit but some are better than others.
You can also use vegtable oil with your diesel ( 50/50 ) to cut down on fuel costs..... the only problem is that your car will smell like a McDonalds restaurant all day long
