Some very helpful advice above; I think I got this down fairly well.
...except for spells/spell-casters...any good suggestions on how to handle these on the fly?
This is how I do it (there is a version of this, and an expanded version, in Crawl! *shameless plug*)
Simple OSR Spell Conversion in DCC RPG
Spell Check: When you cast the spell you have to make a spell check. (Roll a d20, Modified by Caster Level, plus/minus appropriate Intelligence or Personality mod, minus Spell Level x2, minus Armor Check penalty.) Roll:
(d20 + Caster Level + Ability Mod - Spell Level x2 - Armor Check)10 or less:
Spell Fails-lose spell for the day if Wizard. Increase fumble by one if Cleric (as Core Rules.)11 -19:
Spell casts as usual, spell is available to be cast again.20 or more:
Spell casts with maximum effect, spell is available to be cast again.
Critical success and Fumbles:
A natural 1 is always a fumble (sometimes more for clerics.) Fumbled spells are automatically lost for the day for Wizards and the caster must suffer the effect of either corruption or disapproval (see DCC RPG core rules.)
A natural 20 is always a critical success. A critical success results in all spell effects maximized and doubled if applicable.
This is particularly useful when using OSR/Classic D&D spell casters and they have their spells listed that aren't available in DCC RPG.
(a couple edits, because this was cut & paste from an earlier draft.)