We've been down this sort of road on a couple of different occasions here on HBT. Here are some links which you may find interesting:
Some important things to note:
Porter in the late C18 was a "stock" ale, meaning it was left to mature for quite a while in the wood. That imparts a distinctive flavor profile; ask anyone who's brewed with Brettanomyces. The spices and other additions not hops, malt, and sugar are, in my opinion as a brewing historian, meant to mimic the flavor of "stock" porter in a beer which can be consumed without expensive aging. There is another school of thought, that they are to adjust brewing liquor. At any rate, many of the spices and chemicals are dangerous, because some are poisonous. Best to avoid them.
"Essentia bina" is essentially molasses. You can make your own, as you've found here on HBT, but it's a PITA for something which to my palate anyway differs so subtly from regular ol' molasses. When I do this I use molasses and call it a day.
Treacle, however, is a distinctive thing with a distinctive flavor. I'd not substitute anything else for it.
Keep in mind 4 ounces of hops, or 1/4 pound, is not that much for six gallons, especially operating under the assumption that 1790s hops were low alpha and didn't keep well. It's safe to conclude that by the time hops made it from hop yard to kettle, the AA % was quite low. Don't be afraid to use the stated hops schedule; just use low (sub 5%) AA hops.
You can make "colour" by running wort into the kettle and boiling it down. This is a common historical method.
The color won't be "right", at least as you expect modern Porter to resemble. See the above-referenced threads. Don't futz with it; keep it as true as possible to the original recipe and take a trip back in time.
Good luck, and welcome to the historical brewing obsession!