So, it gets a tad complicated and maybe depends how you define an "engine".
It seems as if Pixel copied the really boilerplatey code from project to project, but that boilerplatey code would be stuff like windowing and DirectDraw setup code.
It follows that changes occurred to this code specifically when Pixel needed to make changes to it.
However, there's definitely code that is both specific to and shared across all Pixel Aquarium games - and some other groups, too.
Since I'm not inclined to do decompilations of a ton of different executables, I'm sticking to just looking at reused error messages, since the implication is that the code said error messages from was copied. Failing that, similar behaviours.
There are four major "sets" I've noticed in this regard:
PiyoPiyo set ("DirectDrawCreate Error" & "Push return", always fullscreen):
+ Azarashi 1998
+ PiyoPiyo
+ Ikachan 1.3 (June 2000 according to title screen)
Pixel Aquarium set ("dxdraw" & "error" & "main surface" & "zDWAVE", always windowed, Pixel Aquarium branding):
+ Azarashi 2001
+ JiLJiL (2006)
+ rain (2007)
Cave Story/Ikachan set: Guess.
Interesting oddities:
+ Glasses/Megane (2003) - This isn't branded Pixel Aquarium, but it acts like a Pixel Aquarium game, but it doesn't seem to have the usual error text.
+ KadoAI (2003) - Yet another "Aquarium-like" game that isn't branded Pixel Aquarium. Could be it's own set?
+ Guxt - Post pxtone but before KB/etc.
That said, I think it's safe to say everything Kero Blaster Related is on the same engine - the different projects with it are actually consistent enough in their internals.
However, calling Pixel Aquarium an engine sort of implies more control over the game's functioning and design than it seems to actually have.
The games are very different but also relatively simple, and Pixel's style isn't really to make generalized code (hence enemy code is hardcoded) unless it's really not worth it when accounting for recompile times/translatability/etc. (hence the contents of text scripts are not hardcoded).
It's also worth mentioning that the Organya support in Azarashi 2001 is specific to Azarashi. It's not included in the other Aquarium games.
