Well, how hard could that be? 3.5, at least, seems pretty simple to convert. Classes are converted like I said, and monsters shouldn't be that much harder.
Monster Conversion:
To get OD&D AC, start with d20 AC. Halve anything after 20, so 22 becomes 21, 30 becomes 25, etc. If you're using ascending AC, that's the number right there. If you're using descending AC, subtract the result from 19 (or 20, whichever makes more sense for the edition you're using).
To get OD&D HP, cut the d20 HP in half (rounding up) and cap the results at 40 HP. This assumes you're using d6 hit dice for monsters. To bring the converted monsters in line with d8 hit dice monsters, divide the d20 HP by 1.6 instead of by 2 and the cap becomes 50 HP.
Monsters should only have a few special abilities. Looking at Balor, a CR20 example from 3.5, we'd keep Spell Resistance 28, Entangle, Vorpal Sword, and Flaming Body, and discard all the rest.
Higher-level monster attacks should get nerfed. A CR 7 attack (chimera dragon's breath) goes from 3d8 to 3d6 (reducing average damage by 3), and the Balor's flaming body attack goes from 6d6 to 3d6 (reducing average damage by 10.5). The pattern kind of looks like "reduce attack damage by CR/2" but it's a pattern of only two points, so I can't guarantee anything.
I think that's about all of it. The number of attacks don't quite match up, but I don't think that really breaks the system.
Did I miss anything?