Here is what I found out from someone 40+ that goes to Kato and found success. They routinely recommend certain blend of supplements that are targeted to improve mitochondrion, so these supplements are actually proven mitochondrial nutrients. The label of the product says that it is evidenced to improve egg quality (I haven't found the research paper on this, hopefully it is published in English)
That particular lab-quality blend is only sold within Japan, but here are the ingredients
CoQ10 (Kaneka QH) 50mg - I think you can get this in the US, my Healthy Origins uses Kaneka
L'Carnitine - 200mg
alpha lipoic acid 50mg
The mg stated above is the weight of the purified ingredients themselves, as opposed to the packaging mg.
Here is the website for this product, it is called IVF supplement, all ingredients made in Japan or USA