Should I replace my engine ? Please Help?

I'm going to hazard a guess that you're an American. Because only Americans seem to still be concerned about sludge in modern engines. The rest of the world stopped worrying about it sometime around the early to mid 1980s.

Unless you've been silly enough to use the cheapest and most basic grade of mineral motor oil, and have left it far too long between oil and filter changes, and you almost exclusively use the car for short journeys where the engine never warms up then there should be no sludge.

But your car has a modern design, and most Saab owners tend to maintain their cars well and use semi-synthetic or fully synthetic oils and they simply don't produce sludge.

As for "I only can clean it once again and then I have to start thinking about getting a new engine...", your mechanic is feeding you bovine excrement of the worst kind.

Use a different mechanic. If you like, get the sump (oil pan) removed and cleaned of any sludge as that's where it will form usually minding its own business and causing no problems. After that refill with semi-synth and fit a new good quality oil filter and change the oil according to Saab specifications.

You haven't described what problems the engine is having which resulted in a highly suspect diagnosis of sludge formation, but whatever they are they almost certainly have another and far simpler cause, such as a filthy air filter.

Whilst on the subject of bovine excrement, don't listen to anyone claiming your engine needs Sea Foam. That's another bit of appalling snake-oil which only Americans believe in and which cannot legally be sold almost anywhere else on the planet as the manufacturers are wholly unable to support the claims made for it. I mention it as some other fools will nod sagely at whatever is causing the problems attributed to sludge and will recommend Sea Foam as an instant cure.