Native PSP ISO is impossible without rewriting the entire game from scratch.

If you want a portable Hollow Knight experience:

The reality is that a native, full-featured PSP ISO does not exist because the console's hardware lacks the RAM and processing power to handle the high-definition hand-drawn assets of Hallownest. The Hollow Knight "PSP" Experience

The most viable "PSP-style" experience is the fan-made . This is a full conversion of the PC version that runs surprisingly well on the Vita's hardware.

There is no official or high-performance "ISO" for the original PlayStation Portable (PSP) that provides a "better" experience than other platforms Hollow Knight