Will a 3100 Buick engine fit in a 2002 Buick lesabre limited

Since the vehicle came with a 3.8L, I would say most likely. How much money do you have to dump into a 19 year old Buick that is worth about a grand? Installing an engine not suited for the vehicle requires more than the engine. Transmission, ECM, wiring harness's and other custom work, and the headache of making sure everything can and will work. If this is a project car, go for it. You will spend thousands getting it running again. Frequent some Buick forums for more information, but to me it is a waste of money. If you really like the car, find another used 3.8L engine to match or purchase another used car.

Plus if you're being told the 3.8L has a blown head gasket, it doesn't. It's the intake manifold gaskets. But if you've let it go and you now have a spun bearing it's time to move on. As TDolby said, unless it's a project car, it's not worth fixing! Hope that helps! Jim