Third reality: the original documents are not the very best ones from nowadays perspective and it may be advisable to write an own version of the POH as well as a personal checklist, which is mainly the official one with some added features for things fit in the last x0 years.
Be careful with that....
The POH (as a "Flight Manual") is a document approved by the certifying authority for the aircraft design. Writing your own version, though perhaps entertaining, would not result in a document which had any legal basis from which to operate the aircraft. This theme extends to the checklist provided in the POH/Flight Manual - it is in the "Approved" section of the POH, and as such, would require reapproval if changed.
If the aircraft has "added features" which are properly approved (usually by STC) and installed, there would be an approved Flight Manual Supplement with these installations. That FMS might contain a supplemental checklist, which would be required to be followed.
Though I'm not aware of any regulation prohibiting writhing your own checklist (or I suppose POH), if you are not operating the aircraft with reference to the approved version, you're on thin ice for compliance (ramp check). I suppose if your home made checklist obviously exactly combines the relevant sum of approved checklists from the original aircraft and any approved mods, and the originals were still available for pilot reference, that would probably be looked upon favourably by authorities.