One class that a lot of people seem to want to run in oldschool games that often doesn't have a published version is the bard. It's seen various incarnations over the years, from the multiclass thief/fighter/druid weirdos of AD&D, to the minor-spellcaster skill monkeys of 3rd, to the wizard-like full caster of 5th.
I'll be completely frank, and say that I don't like the idea of 'magical music' that pops up a lot. It seems somewhat more flippant in tone to what I want from my games. So, whilst performance is an element of the character, I see knowledge as being far more fundamental to the class's function. The bard is the person you bring down the dungeon with you because of their hidden lore, so that they can identify the things you find down there. With this in mind, I'm OK giving them some magic-like abilities that reflect their depth of knowledge.
Bards should, to me, be a fairly squishy class that relies on foreknowledge and trickery rather than brute force. No spell slots, but useful tricks. I also like the idea that a bard levels up quickly, representing the way they learn fast.
The bard, then, uses the same framework as the Specialist; the same saves, hit-dice and XP progression. They also get as many skill-points as a specialist, but can spend them on different skills.
Rather than the specialist's skills, they get access to the following: Architecture, Languages, and Sleight of Hand all work like the Specialist's equivalent. In addition, they get the following skills to pick from: Identify, Leadership, Legend Lore, Manipulation and Read Magic.
Manipulation covers the bard's ability to leverage their social skills to avoid fights. It starts at 0-in-6. For each point in Manipulation, reaction rolls to the bard are 1 point better and enemy morale rolls are 1 point worse. Manipulation only applies when the bard can make themselves understood (although it doesn't depend on language); against wild animals, mindless undead and so on it doesn't function.
Leadership covers the loyalty the bard can instill in their followers. For each point in it, improve the loyalty and morale of the bard's followers, hirelings, retainers and so on by 1.
Identify and Read Magic both start at 0-in-6 chance. Using the skill takes a turn, and if it succeeds the Bard casts the spell in question. If the roll fails, the bard can't use Identify or Read Magic on that subject again.
Legend Lore also starts at 0-in-6. It allows the bard to cast Identify, taking as much time as if it were cast normally. Where the casting time is more than a day, this represents extended research; the bard is visiting libraries, consulting scholars and tracking down rumors. This requires the bard to be in an urban area; in the countryside or wilderness there simply aren't enough collections of knowledge for the bard to consult. If the roll fails, the bard gets no knowledge, but can try again; the only cost is lost time.
Although the Identify, Legend Lore and Read Magic mimic spells, they are not magical abilities and are in fact perfectly mundane, the result of the bard's extensive knowledge.
The result of this is a character who has advantages to the social aspects of the game, and who can innately access the kind of knowledge other characters would rely on magic to achieve. They don't cast spells by singing or anything like that (because that would be silly). However, a bard who's put points into Read Magic can cast from scrolls as easily as a magic-user or elf.