In my Solo D&D Adventure — Curse of the Emerald Elixir — I am mostly using B/X mechanics, as reflected through the lens of Adventurer, Conqueror, King.
One part of the ACKs rules that are unchanged from B/X, and not to my liking, are those for poison.
Poison: Poisonous monsters are among the most dreaded that adventurers will face. A character exposed to the poison of a monster, unless otherwise noted, must immediately succeed in a saving throw versus Poison or be killed. The 4th level cleric spell neutralize poison can be used to restore the stricken character if cast within 10 rounds of the character’s death.ACKS 1st Edition Core, p151
You fail to save, you die. It has the benefit of simplicity, but the downside that all poisons are equally deadly. They don't make you sicken, nor do they give time for treatment (except for the use of Neutraise Poison).
As BX/Blackrazor suggests, the existence of the 10 round grace period on neutralise poison implies that the poison doesn't actually kill you instantly (unless neutralise poison is really ressurection). Instead you are incapacitated the moment you fail the save, but there is a short window in which you are, perhaps, saveable. Interestingly the 10 rounds parallels 3rd Edition's negative HP. Nevertheless, if there isn't someone availble to cast the spell (or in ACK's case, use the equivalent Healing Proficiency), then you are toast, whether you were poisoned by a bee or a purple worm.
I decided to take a varient rule that I'd seen suggested in various places — that Poison did damage dependant on the HD of monster (often 1d6/HD) — and adapt it for my own use.
When a character takes poison damage, they roll a saving throw vs. Poison & Death. If they fail the save they are incapacitated. They fall to the ground, writhing and foaming. On that round, and at the start of each subsequent round, they take damage eqaul to the Hit Dice of the monster that poisoned them.
At the end of each subsequent round they repeat the saving throw. If they pass the save, the ongoing damage ceases.
A Delayed Poison has a gap of one or more rounds between failing the save and the start of damage.
A Slow Poison inflicts its ongoing damage at a period longer than a round (e.g. 1 turn, 1 hour, 1 day).
The spell Neutralise Poison (or its equivalents) can be used to stop the ongoing damage at any time before the character reaches 0HP, but has no effect after this point.
This rule makes poison a potentially survivable event even without treatment, but it depends on the poison. Purple Worm venom still does 15HP damage a round, for example, but Giant Bee stings only inflict 2 damage, and may be survivable without treatment. Paradoxically it also means that low HP characters may have less time to survive, even if Neutralise is avaiable.
More importantly for me, this makes Constitution relevant to Poison survival (through the number of hit points you have) which just makes sense to me. B/X loves uncorrelated numbers — High Constitution makes you better at surviving one sort of damage, but has no effect on saving throws, and therefore on another form of damage — but I love correlation, so it is a double win.