If you look a bit further down on the forums, you might see a guide I wrote a few years ago regarding downgrading Spark the Electric Jester to Version 1.0. Although cumbersome, ultimately the method allowed for a complete downgrade using a legitimate Steam copy of the game.

In the years since, the ability to downgrade in this manor was removed from the Steam servers, so downgrading in general if you hadn't already done so (or made a 1.0 backup before the cutoff point) has been impossible using a legit Steam copy of the game. However after some back and forth between the game's mods and LakeFeperd, a new downgrade method has been implemented and it's extremely simple.

OPTIONAL: If you wish to carry over your save file from another version of the game, go to your game install folder and find the file "save.bin". Copy this to somewhere such as your desktop. Once you have followed the steps below, please paste this back into the game's install folder within your Steam common folder (typically found in "C:\Program Files (x86)\Steam\steamapps\common\Spark the Electric Jester").

STEPS:

1. Install Spark the Electric Jester if you have not done so already. If you have a 1.0 copy obtained from elsewhere, uninstall/delete the game and reinstall the newest patch from Steam.
2. Right click the game in your Steam library and press "Properties".
3. Go to "Betas" and on the dropdown select the option for "beta - Original 1.0 Release Version". The game will now redownload.

And that's it, you should have a legitimate and fully functional 1.0 copy of Spark the Electric Jester! Thanks to both ArcaniaCQ and LakeFeperd for getting this sorted out, I'm sure all of you will agree that it's a huge relief to be able to conveniently and safely downgrade to the speedrun friendly version of the game.