That's quite a big effort modification project to do, if you really want to do it 1st thing I'd suggest is work out the budget involved and then see if it's worth just modifying the existing engine to achieve a similar result or if it's more cost effective/less effort to install the SAAB engine instead...
Power output of any of those can be modified anyway, so if the SAAB's engine has a higher output than the Opel, I'm sure the Opel can be modified to similar and/or greater output, so I wouldn't consider using the SAAB engine if it was just based on that part of it.
If you do choose to modify the existing engine, you can also consider other forced induction options eg: supercharging as there are now kits for these or you could stick to n/a and modify capacity (stroke) the engine and adopt higher performance internals...
Either way you should also consider the rest of the drivetrain as well, if you increase power appreciably you'll need to look at the trans and other associated mechanicals to make sure they cope with the added stress, you need to also address cooling and brakes to make sure the whole package remains reliable and safe...also be aware that modification/s of this type will render the vehicle completely unroadworthy unless you go to great lengths to have it all engineer certified to avoid being pulled off the road in the event of being issued a 'canary'...
Unless it becomes a proper 'street machine type' project or 2nd/3rd car it's all these considerations and more that usually force people to buy something else instead that already has all of what you want from factory especially if it's an everyday car, then just tinker with that