HOW TO FIX THIS EPISODE SO YOU CAN FINISH IT:
To get past the "Laddergate" level, just climb up the looonnng ladder to the top.
The next level after that "Back To Safety" CAN NOT be played so you have to add one of the unused level names to the world file
(I used one that is finishable and does not contain a Star).
Edit the "Mario Mania.wld" file,
search for "Back To Safety", change the line BEFORE it from "" to "42 - Ultrasonic Oligarchy.lvl",
save the file.
When you finish "Back To Safety", it opens up the next level on the map then takes you back to "Laddergate";
so you will have to play "Laddergate" twice to get past it. This is obviously not the perfect fix for this episode,
but at least you will be able to finish it.