You could try looking for the cheat code function and re-enabling it.
Hint:
Ha ha it's not there anymore!
There's a sprite table somewhere; if you can find it you can re-arrange the sprites that appear on each level. I'd start by scanning for the attributes of a given sprite, count backwards over its pixel data, then look for its address being referenced in a tabular sort of a way. You could even write an editor if you're feeling generous, but you'd probably be better received if you did that for the free version.
Or try to find the bit that works out how many baddies you have to kill to progress to the next level and make it smaller, not bigger.