I mostly make things up or inprovise, but haven't done any gaming in a while so recently I haven't even done that much.
The Stargrave rules are fairly easy to adapt to a "traditional" mid-war pulp setting, and I expect Frostgrave, Dead Man's Hand, and Chicago Way are, too. By default, Stargrave should be just fine for pulp sci-fi in the style of Buck Rogers, Commander Cody, Flash Godon, and so on, while Dead Man's Hand has a default grounded in pulp westerns. These sorts of games could probably also be easy to adapt to a "modern" setting using Wargames Atlantic's "Pulp Adventure" zombies, survivors, agents, operators, etc., covering a wealth of pulp-style settings from the '60s onward into the near future (e.g. cberpunk, Shadowrun, Vampire: the Masquerade, etc.) Shouldn't be hard to do war-themed pulp subjects (GI Joe, Sergeant Rock, etc.), spies, probably superheroes as well (especially the more self-made superhero types like Doc Savage or the Shadow, the A-Team, etc.)
02-Hundred Hours might lend itself nicely to a broader pulp genre, and seems well-suited by default for infantry-focused World War and even Cold War themed games.
Seems like "The Last War" ruleset might work just fine, as might the related Mork Borg and Forbidden Psalm rules et, with a little tweaking. The Last War's rule set certainly seems simple and nicely suited for short skirmish games with new players.
RPG rules might also work, to at least some degree. I bought a copy of Deadlands for Savage Worlds a few years ago but never played it, I don't remember the rules much now, but have a vague impression that Savage Worlds' rules seemed skirmish-game-friendly.
Chaosium released a Pulp Cthulhu rulebook several years ago, which I've got some mixed feelings about. It IS a beautiful book, and the writers clearly produced the book as a labor of love, filled with a great deal of documentary and "fluff" content aimed toward the sort of pulp literature produced from the 1930s to the early 1940s or so. Not a lot of "crunch" in the book, though, with the understanding that it's more of a setting for the core Call of Cthulhu rules than a stand-alone RPG. The default Call of Cthulhu rules MIGHT not be very hard to adapt to skirmish game purposes, but I never really thought they invited such a conversion; the "pulp Cthulhu" additions to the base rule set are optional rules that mostly just make characters a little harder to kill, which really didn't do much to capture any distinct genre feeling. I'd give it a 5/5 for "fluff" and documentary style, but maybe a 2/5 for crunch "substance" as long as I feel generous. For conversion to skirmish purposes, I kinda feel like the default Call of Cthulhu/BRP could probably benefit from a more stripped-down set of gun and other combat rules, and possibly a more formalized set of movement rules, but the default Call of Cthulhu fules set seems like it shines more for investigation and theatre-of-the-mind roleplaying than it does miniatures combat. The background information on pulp literature and general setting content, however, is fine stuff.
I spent a week a couple years ago adapting 5th Edition D&D rules for a Weird West setting, and it seemed to me very easy to do and likely just as easy to run into Cthulhu-style Jazz Age or traditional mid-war pulp territory; "vanilla" D&D is pretty solidly pulp fantasy in the vein of Conan, Tarzan, John Carter, Pellucidar, Lost World, and other classic fantasy settings. The D&D rules since 3rd Edition seem to have aiming pretty squarely into board-game and skirmish territory, in any event, so I don't see any reason why 5th Edition D&D couldn't be used as-is for a more-or-less pulp-style game.
So, my conclusion: of these options, my dark-horse favorite now might end up being 5th Edition D&D, of all the damned things: I've come to loathe Dungeons & Dragons in its modern incarnations, I've grown disenchanted with its emphasis on miniatures-and-mapgrids gaming over the theatre-of-the-mind use of imagination for roleplaying in general, so I wouldn't expect to enjoy the ruleset itself for homebrewing games outside the "kitchen sink fantasy" genre that D&D has effectively codified as the default "new normal" for modern fantasy. But, it seems like a surprisingly easy ruleset to work with, and easier to appreciate once the usual D&D setting cliches are moved out of the way.
Otherwise, I don't see any reason why any popular skrimish game rules couldn't be adapted with minimal effort, and I might be inclined toward trying The Last War or maybe Stargrave as the base game for this purpose.