This because I have a friend in Windsor Ontario that actually did just that !

( I thought all of the manufactures painted the assembly line engines without primer? )

^^^ I've seen plenty of peeling/flaking OEM engine paint through the years, especially after the engines were a few years old.
Especially on the tin.
But not all of them, to be fair.
Whether it was due to chemical strength or cleanliness (which could be doubtful, don't know, wasn't there), I don't know.
I do know that I don't want to paint an engine (or anything else) twice, so I use a primer.
As a bonus, primers generally require less paint application to get the right final color.
Strictly correct? Nope. Looks great for decades? Yup.

Like All solvent based products today, there's no comparison with older products. Be it Paint, Paint Remover, Drain/Household Cleaners, etc. Many of the components used are not used anymore, are banned/restricted, or substituted with lesser types.

I have my local paint supplier mix AMC Mandarin Orange for me in a urethane basecoat and clear over it for the jewelry look.

would it be safe to prime over existing paint that was applied during a rebuild several years ago, or should i just scrub clean and apply paint directly to what is there ?

any opinion on my question ?

I'd try to clean the old paint off with lacquer thinner or enamel reducer & a red ScotchBrite pad first.
An etch primer is for bare metal, but a regular primer will also aid adhesion.
The issue with previous paint is mostly compatibility with the new paint, so it's best to test an area.

You'd need probably 2 pts for all engine parts. That's $130 !!!!!.
Because what? It's a "tad" lighter/darker then Hirsch/POR-15/Mandarin orange.
Crazy price. And people [censored] about car paint prices.
Another "nail" in-the-coffin for moderate hobby restoration projects.
After some time in that running engine compartment, betcha that "correct"
color will change to a degree.

Is that street or race Hemi paint?