I sent an email to Mr. John M McIntosh(@squeaker68882), the developer of Scratch.app for iOS(Scratch.app existed a while in App Store until it was sadly rejected later) asking if there is a way to personally build the app on my iPhone and iPad. He kindly pointed me where I can get the source code.
I built it with Xcode. At first, the app could just start but any Scratch project did not run. I followed John's advice that probably URL for download the projects might be changed, so I changed the source code and I could finally made it run.
It is very nice that John also published the source on GitHub so that anyone can contribute.