I have been running a group of 3.5e/5e converts and have been utilizing simple ability checks and cantrips that my group really seem to like since they just didn't like playing without them. I have provided them here if anyone is interested.
Thanks for the question.
For ability rolls and skill checks I have my players roll against the Saving Throw applying any ability modifiers. Except in cases of class encroachment. For example if a fighter tries to pick a lock he can make a roll with a penalty of -5 and cannot apply any ability bonuses. This penalty for class encroachment can be adjusted as the Gm sees fit.
If you are interested I have a gaming group called The Order Of The Crimson Die. We are a dedicated Swords & Wizardry Light gaming group and I have a blog that chronicles the play, growth and development of my Ruathain Campaign Trilogy designed for play with Swords & Wizardry Light. I invite you to check out my blog as most of my future contributions to the game, be it classes, spells , races and adventures will be found there.