It’s fairly easy to convert, fortunately, isn’t it?
[Attack Bonus] = 10 - [Attack Throw]
Add +10 to AC to get “target number AC.”
So, a 2nd-level fighter has +1 to hit, and an unarmored dweeb has AC 10. The fighter needs to roll 9+ on d20 to hit (9+1=10).
AD&D 2E THAC0 turns into attack bonus easily, too: [Attack Bonus] = 20 - [THACO]. A 1st-level fighter has +0, 5th-level fighter has +4. [Ascending AC] = 20 - [Descending AC], so aAC 5 becomes dAC 15 (in either case, the 1st-level fighter needs to roll 15+).
I’d just ignore the differences between THAC0 and attack results between the games, because the % chance to hit unarmored opponents for a given combatant (HD or class & level) actually varies by edition.
Although, again, I think only the AC conversion matters, since all editions derive base to-hit chances from class & level or HD.