I regularly extend existing LEGO sets, sometimes referred to as a ReMOC, and I also use various original LEGO vehicles to size my MOCs. To do so, I first build the original set in LEGO and then start the modifications. Sometimes during the process, I will often move to Bricklink Studio to polish the results where I restart the same process. This workflow means that I end up with some original sets in Studio format. You can find these sets here, though these files are often just structures without minifigs, vehicles or extras, I hope these might help you to create your MOC.
Disclaimer: These may not always be an exact match, but it is a good starting point. I try to replace the missing part with the closest part I can find in Studio. I also often include any custom parts with stickers if I used any. To open the parts file and inject the file into Bricklink Studio, you will need Bricklink’s Part Designer. I have a tutorial here that explains how to add a custom part to Studio.
I will make new files available as and when I create them.