I've been running bits of (reworked) X4 for my playtest. What I do for monster conversion is just eyeball it.
So, my PCs were going to be fighting a bunch of troops backing a wood juggernaut from the adventure. The troops were 2d level fighters in the module, so I just grabbed the hobgoblin elite stats, and close enough. The officer was a level 7 fighter. I just gave that character the same number of hp (roughly 40, iirc), two attacks per round, good attacks (I think I did +5 or +6/1d10+1d6+4?), and a good but not unhittable AC--I think I went with 16, figuring heavy armor but no shield since he's fighting with a greatsword.
The Juggernaut had, back in the day, relatively bad AC (6=14 in new reckoning), tons of hp (100), and a totally crazy attack: crushing people by rolling over them; against multiple PCs, they get a save versus Dragon Breath, against one PC, no save, attacks as a high level fighter, 8-80 damage. 8-80 damage seemed a little crazy, so I cut that down to 4d10. The Dragon Breath save became a Dex save, DC 14. And the attack became +6. I kept the AC the same (14), so it's not a totally soft target but pretty easy to hit. And I either kept the hp the same or reduced them a little to 80, I don't remember.
It was a fun, memorable fight imo.
So, I think 1st ed or other old editions is not too far off in power level, but I would just start with the stats you have, and adjust them in a direction that seems fun and appropriate. You're not testing balance at all, really, because the balance depends on your conversion work--so just try to eyeball the balance to your preferred level of danger.
