Did you ask for permission to use the sprites/improve the engine?
Oh boo hoo. He did not improve the engine, it's not even an egine, it's perspective code and layered drawing, he's not even using the original code for the perspective... Look at the other engine... The only thing the code has in common is lengthdir.
I'm glad he emulated the function from the original, the code in there; you can't even grab given it's in a dll for some weird reason.
And layered drawing is not something you can call an original idea. it's been around some.
It's not the first time someone emulates a feature and simplifies if for all to use.
Though using the same sprites, if without permission AND the original sprites are the other engine's SPECIFIC sprites (owned by the creator of that engine) I would say, remove them and use other ones.
Accusations aside, that other engine is very cool.