fmriprep was developed by someone else, you could ask them for help.

The HCP pipelines are our preprocessing pipelines, which now have options for operating on "legacy-style" datasets, for instance not having a T2w image.  We still consider fieldmaps to be very important, as "fieldmap-less" distortion correction of EPI has not been convincingly helpful in our testing.

As another option, you could look at ciftify:

https://github.com/edickie/ciftify

Tim