aerox carb to stage6 streetrace pipe issue need help

hello, so i haad a mechanic look at my bike and he said head gasket issue, i had a brand new 70 stage6 streetrace kit laid around so i bought a new crank from pedparts and am having all of it fitted by a mechanic

anyway my issue is in the old cylinder head (50cc standard) it has two brass nipples which connect to 2 nipples on the 17.5mm carb, the new 70 streetrace head has a hole with a screw in which i guess you put 1 hose into but where does the other go? what do i do??

thanks