There are titanium lug nuts out there, I personally don’t use them, they seem soft and get chewed up easily.
Besides using aluminum tie rods and ends, everything else we use is steel, 95% drilled and pinned. There’s not enough bolts on a kart for a huge weight savings, different wheels weigh less than others as do gears, sprocket guards and hubs. We add weight to all our karts, so I go for strength instead.
I was working for a sprint car team a couple years ago. Discovered a bent frame rail, during maintenance, after a wreck the night before. We were thrashing to get the backup car down and ready. It had three grade 8 nerf bar bolts on either side instead of the normal ti bolts we ran. Prominent driver asks where the ti bolts are because he doesn’t want the grade 8 in there. We’re talking about six 10-32x1 1/2” bolts. Talking grams here. Logic and racecar spending rarely go together ???