I want to build http://www.hobby-hour.com/electronics/lm2576hv-adj-buck-regulator-schematic.png

From http://www.hobby-hour.com/electronics/lm2576-step-down-switching-regulator.php

I might not be able to source exactly the same parts. I assume I have to use the same IC, but could I use a, say, 200uH inductor instead of a 100uH? Or slightly different types of capacitors, with different capacitances? How about that Zener diode?

If I'm stepping down 24 to 3V and drawing up to 400mA, I'll probably still want to attach a heatsink, no?

edit: the output voltage will be running a small 3V gear-motor system needing to lift a small amount of weight (half to a couple of kilos) vertically in a reasonable amount of time.

 

It's really not a good idea to substitute parts randomly without having some idea of what the compromise involves. Circuits can cease to function or become unstable with spectacular results.

 

Download the data sheet. It will give you data on selection value of different items and you can design a circuit based on the values you can find.