This has been a topic of discussion on the RCT1 forum as well. It should be noted that using a pre-built ride, other than the tracks that come with the game, is prohibited during runs. While you are allowed to make a track during the game and copy/paste that throughout the park to manipulate whatever you're trying to manipulate, using a pre-built track of your own design gives an unfair advantage.
Unfortunately, this invalidates both Marcel Vos and Tappajav's runs, as both use a custom pre-built track design, but since neither are members of the site, I doubt they'll be too broken up.
I received a DM from a user asking if this applies to Full-game runs as well, if you can make a custom track in the first map and then continue using it for the rest of the game, as opposed to building a new custom track in each scenario. This is accepted, you are still making the track during the run, and is an element of regular activity for normal playthroughs of the game.
I wasn't able to contact Sko about this as they don't have social media linked to their account, so if they want to chime in on this topic, we can figure something out.
I thought I had my Discord linked to this. I just linked my Discord and YouTube now. Also, in addition to the Pre-Built design discussion. No preparation before the timer starts is allowed. This means all saved designs before the timer are NOT allowed. If you start at Crazy Castle (for example) you can create a design or use a Game Developed Ride Design and save it. Then in later scenarios, you can choose the custom design (The one you created from Crazy Castle) to build. If you choose to build your own custom designs, you have to make and save them for every new run. Hope I made that clear for everyone haha.