If you can do such modif as long as you have money, then OPM universe would have tons of wealthy individuals getting modified into cyborg, which is not the case.
There are only a few people in the OPM universe who can do such modification. We've only seen 2 of them so far.
Genos has Dr. Kuseno, Armored Gorilla has the scientist from House of Evolution who is talented enough to create a Dragon-level being like Carnage Kabuto.
So while the universe of OPM is capable of superpowers and futuristic tech, MOST of these techs were never shared with outsiders by the creators.
Even if Iaian wanted a partial modif to attach a new arm, he would have no idea where to find someone who can do that.
@Lucky_Jay @Liquidxlax
TLDR; Imagine a world where new discoveries and inventions are not shared with the rest of the world, but kept for personal use. That's what OPM is like in terms of tech, some genius individual invent and thus has access to 2199 tech, but the rest of the world are still living in 2019.