Back in the day of playing with the Xono, I found the perfect low noise LEDs. They are a very strange package, they are leaded, but the leads aren't under the bulb.. think axial LED. They are super expensive (for LEDs), I think about $1.50+ each. I sent a bunch to Scott Wurcer and John Curl a while back, and one of them did some tests on a Audio Precision 1 and said they were about as good as you can get. I can see if I can find them again, but I'm not sure they'll fit very easily. I also have a bunch of transparent orange LEDs that are dead on 1.7V that I used in Kevin's other designs, but sadly they are no longer being made.
Ok, I see C31A now. I think 220pF will work here, that's what I'll prototype at least.
I'd like to revise my suggestion for R19/R20... at minimum a pad here for prototyping. Ideally, a way to connect to FB or to ground with minimal trace length.
I think the MK3 are rather nice in fact, but you'll really have to change things around a lot due to availability. Just calculate things as ratios and you'll be fine.