I'm rebuilding the e61 group head on a grimac la uno, after removing all the scale I noticed some of the chrome plating in the mushroom and on the inside of the group is flaking. This parts are constantly in touch with the water and not visible from the outside so I'm wondering if it would make sense to just polish down the remaining plating and leave only brass exposed? Would there be any downside?

Is it new enough to be "lead free" brass?

Probably not, I believe it's probably from 1998, I think lead regulations came a few years later. I was reading this other post about handling lead brass in machines (Addressing Lead in pre-2014/2004 espresso machines) and thought maybe thats what I'll do after removing the remaining plating left

Why not just buy a new stainless version?

That's an option, just hoping to reuse as much as possible since it's still in good shape other than the plating, I might attempt to diy a nickel plating since I already got the supplies needed, but if that goes awry I'll just order a new one

water from my e61 group head had terrible iron aftertaste. That was the reason I sold it after a short time and never looked for another e61 machine. Too many metal parts of various finishes that water travels through until it reaches coffee. Manual Levers are probably the best option in that regard