My dads 2003 Buick century has just hit 200,000 miles and the engine has been suffering for awhile now. I love the car but today the power is very low. I was thinking about buying a salvage Buick regal LS or GS and swapping out the engine and other things that go along. I was leaning twords an LS because the transmission is the same so I could keep the column shifter. To my understanding this swap is pretty straightforward if you go the LS route and have a 2003 and above century. I looked but can't find any one who has actually gone forward with the swap. Any info would be much appreciated!

All Series 2 3800'* manufactured from 1995-2005 are the same. Aside from different brackets used from platform to platform. In other words, you can get any 3800(obviously the new the better). Just swap all mounting brackets and all of your sensors. They do change from year to year.

Addressing why the engine in the vehicle now is losing power would probably be easier than putting a new (used) engine in. Better the devil you know than the devil you don't. What symptoms are you experiencing?

I should have been clearer! Its not losing power it just feels slow compared to newer cars. I am still thinking about rebuilding the current engine but I always wanted to put a bigger engine in.

my friend has 1997 regal GS and next time I have to do work on his car I will measure engine cradle mounts. I'm hoping I can just swap the cradles.

Cradles are also different across the platforms.

My dads 2003 Buick century has just hit 200,000 miles......Its not losing power it just feels slow compared to newer cars.

14 year old car with 200K "feels" slow compared to newer cars? Really?

​​​​​​thats what I said...
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What do you mean by "feels slow"? Acceleration response time? Have you tried any basic tune up procedures? As i'm sure you know, it could probably stand to have cleaned the MAF, the throttle body, fuel injectors, spark plugs, etc...