Yes, either a junk yard used one or you can purchase a crate engine, that is just the main engine block internals and heads, and you swap all your parts to it.

Currently doing an engine swap on a friends car.

Depends on the car, I don't think it's too hard to find one from a scrapyard

Sure. Basically every part of a car is replaceable, including the engine (and parts of the engine). As the car gets older finding the parts gets harder but that's about it.

Engine swaps are expensive though, especially since you wouldn't be doing it yourself knowing next to nothing about cars, so a lot of people get a new car at that point.

But there's nothing stopping you.

Yes. I bought my f150 for 2k not running. Replaced the engine (my labor, 4k worth) and now I have a 15k truck.

If you want some side cash flipping old cars can be profitable.