Inosine is interesting. Banned for athletic competition (although that's not proof), and the few studies I found since Jensen continue to support the correlation although none of them are proper RCTs of its effect on cognition.

The downside is that it looks like it does increase your risk of gout/similar problems, so something of a tradeoff there...

I've been trying out low doses of inosine myself (just temporarily, I don't intend to replace the bottle when I runs out because of the gout/joints issue), and maybe there's an effect? It does seem to mess with my sleep the way other stimulants like modafinil do, at least.