If you asked this question knowing almost nothing about the Japanese writing system, you should read a bit about it first. 腹切り is a perfectly valid Japanese word, but it's written using a type of character called kanji, which are ideograms. 腹 is a character that represents the meaning of "abdomen", and 切 is an ideogram that means "cut". Although the kanji 腹 has several standard readings including "hara", you cannot use kanji to explain the pronunciation of a word. As an analogy, you can use the emoji 🐱 to convey the meaning of cat, and it has a standard reading (/kæt/), but you cannot use this character to explain the historical pronunciations of the word. Even if it was pronounced differently in the past, 🐱 is always written as 🐱.
If you want to compare various pronunciations of a word, you need to use hiragana instead, which are phonetic characters similar to the Latin alphabet. Here's how you can simulate the old pronunciations of "harakiri" using hiragana:
Note that the latter two rows are just rough simulations. No one would understand you if you pronounced or wrote this word in these old ways.
(By the way, the word-initial "p" sound in Japanese words disappeared before the 8th century, and the practice of harakiri did not appear until the 12th century. I doubt that people in the past actually pronounced it like "parakiri".)