Car Engine Replacement?

You can feel that way about it and it will still have periodic trouble.All cars break down.

....a new car with all the squeaks, rattles, sagging driver's seat, cracked glass, loose handles, burnt out speakers, stale odor, sloppy steering....

No. There are tons of other parts to a vehicle that effect how it handles and maneuvers. Suspension, wheels, brakes, and steering. 

It would likely operate a lot better and have more power and easier shifts, but you will still have older springs, shocks and brakes, seats and everything else.  Decide if your car is comfortable enough and rides good enough to make it worth the mechanical investment. 

And of course if the body is a rust bucket, its probably not worth it.

it would be like having a new engine in an old car ..

Not at all, you still have the old rear end the old brake system, the old gas thank: all you have is a new engine and transmission every thing else is old crap!

No---------------------------------

No, I had a 1978 Toyota corolla and Put a Japanese Imported Engine in it in about 1990

the engine had 38,000 Klm and even with  Celica 5 speed Gearbox  it still felt like a 1978 Toyota Corolla

You'll still be using the old body, interior, and parts under the hood.

NOW really! In reality, most engines and trannieis will last as long as YOU want them to last, if you use modern SYNTHETIC FLUIDS in it and CHANGe the fluid in the tran at about 80,000 miles! The OTHER iSsues are BRAKE LINES and HOSE failures, SUSPENSION po[roblems, Tie rods and tie rod ends! BALL JOINTS and BUSHINGS along with EXHAUST systems and CONVERTER issues! ALL modern engiens will EASILY get to 200,000 and DOUBLE that with MODERN SYNTEHTIC OILS! Steeriong is also a biggy like the PUMP and the RACK and PINION< etc! Many people replace the entire engine when only a HEAD GASKET has failedor a mild OVERHEAT has stretched out the HEAD BOLTS! They are cheap at about $30 or so! Pelp using WAX BASED OILS i or ASH bassed oil like VALOVLINE will need new seals and gaskets! If they used SYNTHETIC OIL like PENNZOIL ULTRA PLATONIM< this woudl NOT happen as PENNZOIL wil PROTECT seal;s and gaskets and RUBBER PCV HOSES as well and R PRESERVE the O2 sensors and the converter