I have a 75W mercury lamp (H43) that I am currently running on a 50W (M110) MH ballast with no ignitor. The lamp is very bright, but it is new. I have tried looking up specs here and there to find some compatibility, but I noticed the MV lamp has a 130ish arc voltage, but the MH one has an 85ish arc voltage. The arc current is the same at around 0.6 A.

I don't really want to overheat my ballast, but I'd like to run that lamp. It is currently in a lamp post, with a remote ballast in a weather proof enclosure. If a M110 ballast with no ignitor is not appropriate, would any of you have suggestions to use as a substitute ? There is a postline on eBay but I would rather use my existing enclosure than build something new. Also, I might want to use MH in the future.

Many thanks.

Can you measure the lamp and line current, how hot does the ballast run compared to with a mh lamp?
Does it have a capacitor, if so where is it connected?

I guess the ballast is HX.
Then the higher arc voltage actually means slightly lower current so lower ballast loading (dont let the higher real power fool you, what matters is the apparent power handled by the ballast and that gets slightly lower when the arc voltage increases).
So I wont worry about the ballast at all. For the lamp it is a question, what is the real lamp current in your setup vs what the lamp is rated for, if the real current is not much (= more than 10%) above the lamp rating, it should be fine as well.