automationgame.com

OK so I tried to uncheck the open beta tag on Steam so I could open some bugged car designs that work on non-beta.

Well, after downloading the "regular client", the game won't start. Fatal error regarding some factory stuff.
Do I need to do a clean install? And if so, how do I make sure I don't lose my car and engine designs?

I am guessing you uninstall, uncheck beta, and then clean install.

That happens when you make new cars on the beta (and possibly if you recalculate a car you already made). That's because the way the open beta handles the cars, they all load in a database instead of loading each file separatly AFAIK, so when you switch from open beta to stable release your asking the game to open the open beta car files in the stable release way.
The easiest solution is to re-name the Automation folder in your documents and re launch the game, that will create a new folder. Then you can pass the lua files for your cars one by one until you find the one making the comflict.

An advice: when you launch the open beta, under the options menu (where you choose the language and the resolution) you can create a new save path for the open beta (something like C:/games/automation) that way you will have a dedicated folder for the beta and one for the stable release, that way the error won't happen.