Reminder, helm refers both to the place a vessel is steered and the person performing that duty, although helmsman is also used and Spelljammer refers to the person operating a spelljamming helm. In Spelljammer, a helm is a magical chair the operator sits in. It is not a piece of headgear. All three of the iterations of SJ helms assume Vancian magic, as do my current rules.
In 2e SJ, the two standard helms, minor and major, have a limit on the size of craft they can move of 50 and 100 SJ Tons (100 cu yds) respectively. They also translate the caster level of the helmsman to Spelljammer Rating, the number of hexes a ship can move at the rate of 1/3 and 1/2 respectively. Ships also have a Maneuverability Class that determines how sharply the ship can turn per hex. There is also the much complained about helms drain the spellcasting ability of the helmsman.
In the 3e SJ mini-game/setting Shadow of the Spider Moon, helms work slightly different. The speed of a ship is determined by the size class and quality of the helm, minor or major, with an upper limit placed on the minor helm. Maneuverability is still a quality of the ship not the helm. These helms don't drain spellcasting, only requiring a spellcaster to operate.
Now there are rules for helms in 5e...hidden in a fucking non-spelljamming adventure...thanks a lot WotC. Anyways, the speed of those helms is based on the highest unexpended spell slot. That's kinda cool. It's like the 2e helms as it bases the speed on the caster's level, and it's like the 3e helms by not draining spellcasting for the day.
These are all specific to combat speeds. There is also spelljamming speed which is really really fast and either the same regardless of the helm or who is piloting, or with a slight variance, favoring major helms.
Translating these rules to SWN is a bit tricky. For spike drive equipped ships, which is the default assumption, speed is comparable to maneuverability class, which is a factor of the ship hull, and is used for combat; while the drive rating is used for in system speed and the distance a ship can travel via Drilling, i.e. SWN FTL. The inverse of how helms work. I could have stuck with that by having helms determine the drive rating by helmsman level in some way with limits on the size of hull a helm of a particular quality can move. In a generic conversion for the default setting with FTL I'd probably go with that. However, as the setting I'm working on, once called "Orbital Space Refuse" but now "Countless Stars But You're Fucking Stuck With One," is a single system STL setting, I'm using the system ship rules in Engines of Babylon and speed is used for both combat and long distance travel. Which leads to my spelljammer rules for SWN.
Each hull class has a number, 1-4 for fighter through capital-class hulls.
Helms don't drain spellcasting for the day, but they are based on the highest unexpended spell slot, with a level softcap of 10 means 6th level spells at the highest.
Ignore the speed of the hull, an active helm overrides that stat because magic. Instead the speed is determined by highest unexpended spell slot level divided by hull rating for a major helm, and divided by twice the hull rating for minor helms, all rounded down. 0 is a valid speed for a ship to have as unintuitive as it may be to someone unfamiliar with SWN. Major helms can move any class of hull. Minor helms stop at frigate-class hulls.
A helmsman can operate a helm for 12 hours with no ill effects. Every additional 12 hours expends the highest unexpended spell slot potentially reducing the ships speed. Piloting in combat is considered 12 hours regardless of how long the combat was, but any adjustment to speed occurs afterwards. Because of these limitations, spelljammers will either have multiple casters to pilot in shifts or only operate the helm for a fraction of each day, commonly 6, 8, or 12 hours, reducing the ship speed by 1/4, 1/3, or 1/2 respectively, rounding down, if this reduces a ships speed to zero, the travel time also increases 4x, 3x, or 2x respectively. No spellcasting is possible while operating a helm.
As befits them being extremely advance magi-tech they take the form of ornate chairs and are essentially plug-n-play. Just find someplace to bolt the helm down and set a spellcaster down in it. A minor helm costs roughly 100,000 credits, and a major helm costs roughly 250,000 credits. I'm lazy and just used the prices from 2e SJ. The rarity, value, and portability as it stands does make them easily stolen, therefore prime targets for space pirates.
A rough outline for converting spelljammer from Spelljammer is to compare their minimum crew to a particular base hull. It turns out that in general the ships in the original box set are generally frigate-class hulls, mostly patrol boats or free traders. Another rough guide is to compare the largest dimension to either the Millennium Falcon or the Serenity, both of which I would classify as frigate-class hulls, free traders (TL4) or passenger liners (TL3).
As my setting assumes system drives are the default, this also helps solve the "elven flitters are trash in one of their listed roles as fighter craft" because a military force isn't going to put high enough level casters to make a flitter fast enough to run down larger vessels, and conventional drives are in many respects better than a spelljamming helm. The advantage of a helm is no fuel, and the advantage of a system drive is higher base speed and not needing a wizard to operate it.