I don't know what federal laws you have to satisfy in order to construct a firearm from scratch (not parts) but I cannot think of any Hawaii firearm laws that says you cannot mill and build your own firearm from scratch. Yes you would have to develop a serial number system and ifyouwere going to sell any you probably need an FFL? and all the right tax license/business license.
Ok, found that there is a state license you need to manufacture.
§134-33 Punishment for violations of section 134-32. Any person who manufactures or sells any firearms within the State without having a valid license so to do, or who being a holder of a license violates any of the terms or conditions of the same, shall be fined not less than $100 nor more than $1,000 or imprisoned not less than three months nor more than one year. [L 1921, c 13, pt of §1; RL 1925, §2036; RL 1935, §2557; RL 1945, §7198; am L 1953, c 155, §1(c); am L 1955, c 54, §2; RL 1955, §157-33; HRS §134-33]