I've seen some cracking engine conversion in these cars over the years when I was a regular member of a few Vauxhall sites. There are many, many roads you can follow with these cars depending of corse how far you want to go.
To give you an example of a couple, they can be really easy conversion with straight swaps with a the X18XEV and the X20XEV (1.8 16v and 2.0 16v eco-tec engines) and using these will mean that you only have to source brakes suspension etc from the similar car the engine came from i.e. if you were to drop a 1.8 from a Vectra, you will be able to use the brake set-up no probs as they are the same offset and stud pattern
And then there is one that I have seen that is just mental !!!. This is what the chap has done.
Buy 1 Calibra Turbo 4x4.
Buy 1 Vauxhall Tigra
Buy 1 Vectra GSi 2.5v6 24v 196bhp engine
Chop the Tigra in half and make entire chassis the exact width of the Calibra Turbo and weld back together.
Then take the 4x4 system from the Calibra, shorten the front-to-back drive shaft and fit to Tigra.
Then take the 6 speed gearbox, 288mm brakes and the suspension from the Calibra and fit to tigra.
Then drop in the 2.5 V6 engine (mates perfectly to the 6 speed box with needing no modifications. I have done this myself to a friends Vectra by dropping a 6 speed into it
)
Then you have a massive wide wheel based 2.5 V6, 6 speed Tigra that will go like the clappers and handle like a rally car
You could also consider the good old Red Top engine which is a common conversion or even if you are feeling daring and have the money to uprate everthing else to handle the power, you could go for the Red Top Turbo engine. I used one of these in my Vectra Conversion and was a great success.
Hope this helps you a bit