Module to Old School Essentials

Have a mode to run a campaign in d&d b/x or retro clones in Fantasy Grounds?

Ascending Armor Class and other options?

I would look at the 2E ruleset.

In my opinion, the easiest way to play D&D B/X or OSE in FG is to use damned's MoreCore ruleset. If you search the Digital Dungeon Master's channel on YouTube, you will find out how he was able to start a BECMI campaign thanks to MoreCore. By the way, the forum user Old Scouser Roleplaying has created a MoreCore extension for OSE. You can find it here. But you can also easily set up an OSE campaign without such an extension. Just use MoreCore. It's very flexible and well supported.

By the way, here is a screenshot of my personal implementation of OSE in MoreCore. It's not very advanced yet, but now that I know how to set up the different rolls in MoreCore, I think I should be able to start an adventure relatively quickly. I don't use anything else but MoreCore and an OSR theme whose download location I currently cannot find.

I am starting to create a small B/X adventure using OSE also. It appears quite doable and I was able to run some basic combat against some Gnolls and Goblins. I am playing a 5e solo module and it is very well done and FG makes solo combat for me much easier. Still, the simplicity of B/X is the way I plan to go. I do have to build everything but there isn't that much needed on day 1 of an adventure and once it is done, it is done. I am having trouble creating encounters though which is annoying but not a deal breaker.