T-shirt printers accept bitmap images. Ask them the needed pixel dimensions for highest available accuracy and how to proceed to keep the colors.
Then you must decide how much you want to smooth the roughness of the original.
An opinion: I think no fixes are needed except increasing the contrast so that the lightest areas are close the white. Even the tile seams could be kept. As said, it's only my opinion.
Of course, no glossy reflections are allowed. One of pro photographer's skills is to avoid glosses already in the photographing session. Your version happily is so good that the glosses can be faded by editing.
The next snippet has got that contrast boost and some smoothing.
The colors are still in the CMYK-printable range (checked). The tile seams + discontinuities on them are faded by copying stuff from other parts of the image. The impression of the original non-uniformity of the brush strokes is tried to be kept.
The edge frame is simply repainted with long straight brush strokes. I think this amount of smoothing is too much. But that's my opinion.
Making the same edits for the whole image in Photoshop takes easily half a day assuming one knows well what he does. For a beginner this is an extreme challenge.
Printing on fabric fades details. Photoshop's filter Cutout makes everything coarser. It can compensate beforehand the apparent detail loss:
I do not recommend redrawing the image, but that's also only an opinion.
Not asked, but publishing a replica of someone's artwork does not become more legal if the intention is to collect money for something good, better than for filling copier's pocket. Negotiate at first with a lawyer.