Well, Scott.... I will give my two cents here at the risk being admonished by those with advanced degrees of which I have none. Last year I was 3 for 4 winning "discussions" with degreed professional structural engineers, but that was with regard to large concrete structures, not aircraft. So far this year I am 1 for 1.....I'm on a roll...
.....or due for a smack down...one or 'tuther.Here is what I would do upon initial consideration - Insert the oversized bolt and install nut - torque it correctly and fly the airplane. I know there is less "area" of the fitting (larger hole), but I do NOT think there is a degredation of either bending moments (of that attach fitting), shear capabilities, or (longitudinal) tension/compression loads given proper washers and torque. Regardless, it would take using finite element analysis(to minimize error and increase predictability) to determine that, as any "ultimate load" to failure (compared to John Thorp's calc'd or demonstrated values) would be too close to call IMHO, given the variables in building.....Friction plays a roll in that fitting as well.....YMMV and all other disclaimers apply here, but that's my story and I'm sticking too it.....until proven otherwise by those smarter than I, of which there are many.
If I were to apply a fix to it, an epoxied doubler (with or without rivets) would be what I would do - thickness unknown, but that's where I imagine Bob's calc comes into play. The reason for the epoxy is to give 100% contact and therefore load transfer/distribution back to where the loads have been pre-calc'd. Edit - use as little epoxy as possible (it is a polymer/plastic after all) but get 100% contact - use the clamp and squeeeeeeze method.
Structural guys must have a way of calc'ing this out, and it might not be too complex either - beyond me though. I commend you for being up front about it - I think a lot of builders might have been laise faire with a case of the effits and said/noted nothing. But I seriously think you are fine to fly - keep the G meter less than 6.....I keep mine below 3.5.