Use Gods & Monsters adventures in old-school clones
In the rulebook, I note that Gods & Monsters is compatible with AD&D 1e and 2e adventures. I use a lot of old school adventures in our game, and I see no reason that the Gods & Monsters adventures I’ve written shouldn’t work fine with 1e, 2e, OD&D, B/X, and all of the wonderful modern clones.
The terminology that Gods & Monsters uses sometimes differs from the clones, but it should be pretty obvious what they mean. Reaction rolls are like saving rolls. Survival points are like hit points. That alone should get you through most adventures pretty easily.
Where are the hit points?
Survival points in Gods & Monsters work pretty much just like hit points. The main difference is that characters neither go negative when they run out, nor do they automatically die. Once a Gods & Monsters character runs out of survival, they start gaining injury points, and each injury brings with it the possibility of death. For the most part, injury points are just like negative hit points if you play with that rule.
Because it has injuries, save-or-die effects in D&D are save-or-gain-injuries in Gods & Monsters. So when a description says to make a reaction roll or gain d6 injury points, for example, that’s pretty serious. It’s not just a matter of converting to a d6 hit point loss. It’s really more like save-or-die. But because I have the injury mechanic, I use it somewhat more often than I would use poison or other save-or-die effects if I were writing the adventures for D&D.
You’ll want to, on a case-by-case basis, choose either “save or die”, or “save or be unable to act effectively for d6 days”, or perhaps “save or lose a whole lot of hit points”. In the latter case, each potential injury point should probably equate to about six hit points.
Most creatures only have survival points; special characters (such as player character and major non-player characters) will also have verve. For conversion purposes you can just add survival and verve together for warriors, and ignore verve for non-warriors. Non-warriors in Gods & Monsters can’t use verve for combat except in special situations. You shouldn’t see verve stats very often in adventures, however, since it’s mostly for player characters.
Spells and spirits
In Gods & Monsters, spell and spirit levels track character levels. A second level sorceror can use second level spells. To convert a Gods & Monsters spell level to D&D, halve it and round up (or down, whatever you feel like). For example, the fourth level Gods & Monsters Invisibility spell is equivalent to a second-level spell in D&D.
Combat rounds in Gods & Monsters are ten seconds long. This mostly matters for spells, as anyone translating between B/X and AD&D knows. In AD&D, for combat spells, leave the rounds alone. For non-combat spells, you’ll have to go on a case-by-case basis whether you want to upgrade minutes to turns (so that they last longer than rounds); and whether you want to downgrade rounds to ten seconds. Or if there’s a similar AD&D spell, just use it instead.
In games following the B/X tradition, rounds are already ten seconds, so no change is necessary.
Attacks that are carried on other attacks, such as poison on a blade or disarming an opponent, usually require a “called shot” in Gods & Monsters. In both D&D and Gods & Monsters, a successful attack doesn’t normally mean that the attack actually broke skin; the hit point/survival point systems are very abstract and don’t map to physical flesh. So for “carried attacks” to work, the attacker has to really hit, not just virtually hit. In Gods & Monsters I require that they make a called shot to do this.
A called shot is a normal attack, but at a penalty of 3 to hit; the victim is usually allowed a reaction roll (saving throw) to avoid the carried portion of the attack. You can just ignore the attack penalty and go with the saving throw, since that’s how D&D handles such attacks.
There are six reaction rolls in Gods & Monsters: Health, Evasion, Fortitude, Perception, Reason, and Willpower. There is no one-to-one correspondence between these and the five saving rolls in AD&D. However, it should be pretty easy to choose which AD&D saving throw to use depending on the source of the saving throw (a wand or a dragon, for example) or the effect of the saving throw (petrification or death, for example).
As in D&D, the ability of characters to do something depends on either class or abilities more than on a skill system. I call this a “Dumasian” system, in that skill is innate; it’s the same thing that, in old scifi stories, has people as natural scientists due to their intelligence, or natural athletes due to their innate physical prowess.
As in AD&D 2, however, Gods & Monsters does have a skill system. However, it’s not a one-bonus-per-skill system; skills are part of a field, and the field is what has the bonus. So a character might have the field of “Gambler” at +2, and the skills “carouse”, “poker”, and “bluff”. When trying to carouse, play poker, or bluff their way through a gambling-like situation, they’ll have +2 on their ability rolls. And if they learn a new gambling skill, they’ll immediately have it at +2. Because they’re a gambler, and once they pick up a gambling skill, they’re good at it. Just as in those old scifi stories, if the hero is a great scientist, they can learn chemistry and be as good at it as one would expect a great scientist to be.
There is no requirement that a character have a skill in order to perform a task: skills just improve what they can already innately do as heroes.
If you don’t use skills, you can ignore them; if you use a 2e-style skill system, just get rid of the fields and put the bonus on each skill. And if you use the AD&D skill system, just get rid of the skills and keep the fields—they’re basically professions.