That art style was also used on the Takara-associated TMNT trading cards, but unfortunately, I've never seen an artistic attribution. Too often, those promotional art pieces were simply attributed to the company, rather than the artist.
However, here are my complete scans of the Super Famicom Mutant Warriors manual. Maybe you'll find a clue there:
https://drive.google.com/file/d/11WQ...Nq7pKw0gn/view