The HCM trail as it was isn’t a trail either, it’s more of a vague direction through the jungle the VC used to ferry men and equipment North to South, it was by necessity vauge, mobile and largely inaccessible.
Sounds like a great idea but I think you’ll struggle to find a map per se.
Edit (and don’t forget you’ll be crossing into Cambodia and Laos for at least as much as is in Vietnam if you’re doing it “right”)
another edit (and a lot of it remained hidden by virtue of being classed as impassable by the US and the South Vietnamese. That they got people across, let alone field guns, mortars and vehicles is on par with the Berlin airlift for man made miracle. It might not be so good with a bike!)