If you don't get a satisfying response this time around, post again, especially during the beginning/middle of the week (and during the thick of the semester). A very large number of the historians who make this subreddit so great are actual history professors, who very much seem to enjoy contributing here during work hours. I intuit this because the weekends are... well... rather slow for this subreddit.

I'm certain someone here would be quite interested in your father's work.

I agree with cardboard.

I have passed it on to a few people, but I think you should approach your local university and speak to the history and art history departments about this. Its an amazing resource and those people can help him/you make the best of it.

Maybe ask a local university's language department?

OK, I'm on a phone on a bus with erratic wi-fi, so this is short until I can come back and edit in more on a keyboard.

Anyway, this is my baliwick, as I am a translator (but no Italian). Historical translation is really tough, because you need someone who a) is highly bilingual in your language pair b) understands the historical context and c) can actually write well. I'd suggest looking for a history department with scholars specializing in this area first, then maybe an English editor second to improve the English. Also be ready to pay through the nose. Translation is by the word and more difficult = more expensive. Someone who knows their salt might go with $1.50/word or more, depending on how much of a pain the research looks to be.

Edit: Did promise to come back with a keyboard, but really, the above mostly covers it. You might be able to find a literary translator that's dabbled in it a bit by checking your local translation association, but from what I understand, translation of historical documents is usually done by a historian rather than a translator, and rather piecemeal--that is, the relevant sections only, rather than the full document.

The best way to do this is probably to find a large university and contact their history department. This seems like it would be a great resourse for research

A lot of people in the Historical European Martial Arts community would be very happy with this. What I suggest is to post your inquire on the forum of schola gladiatoria at www.fioredeiliberi.org and I am sure you will get excellent suggestions and even a volunteer or two.

While you have gotten a lot of really great advice on how to get this work out there and available, I just wanna say that this is extremely impressive work and he should be proud of it.

Very neat. Have you tried posting in r/translator

How about if you have it machine translated and then proofread by a qualified person using something like Amazon Mechanical Turk https://www.mturk.com/mturk/welcome ? You wouldn't have to do it all at once.