I replaced my 18 tooth gear for a Jeep 23 tooth gear. It matches up pretty well but the slot for the retainer clip is too small. Any problem with leaving it off?
Yes it will be problem. Buy the proper one that is matched to your gear ratio from Ford Racing. It's that simple. Don't go jimy rigging stuff. It will catch up with you in the long run.
I've never left one off; I suppose you could try it and see. But if you look up the word "retainer" in the dictionary, well, I think there's a reason they call it that. The gear's plastic - I think I'd rather take a small file/awl or some such appropriate tool and try to open the slot up a bit so the clip would fit.