I have an Olympus OM2 film camera, circa 1975. The plastic "selector lever" that switches the camera to manual, auto, and off is broken. If I buy another camera for parts, is it easy to remove the lever and transfer it to the other camera? Do they just pop off and on?

It's not quite as easy as that, as a professional model they were built to last. The entire top plate must come off and it can be a fiddly job needing special tools. OM2's are plentiful and relatively cheap (or even free if you ask around/use FreeCycle) though, so you'd be better to get another working example and keep your original one for spare parts. Try to stick to the OM2n, it has a number of internal improvements over the OM2 and is a more reliable model partially due to the improvements and also simply due to being less ancient than any OM2. Avoid the entirely different and unreliable OM2SP though.