Look, the idea isn't to come up with a precise definition that is immune to edge casing. I'm ok with excluding questionable entries. But there are clearly "combos" that are much too specific to be called an "engine". We can just look for those.
Most of the combos to one degree or another can substitute cards:
Tac/Vault is a nice clean way to start with 10 card hands and generate province buys using discard for benefit. Well the Vault can be replaced by Secret chamber or Storeroom provided that either comes with some live draw (e.g. Lab/Secham/Tac). Likewise, a remarkably similar setup can be observed with 5 Alchemists on deck top, allowing Alchemist/Vault to play 10 card hands and generate provinces using discard for benefit. Other assured draw of big hands (e.g. Scheme/Hunting Party/Village) or top decking Border Village/Apprentice every turn can setup a very similar mechanic.
The pins are highly degenerate. I mean first off most of the "Discard down to 3 cards" attacks are interchangeable; Outpost also can go into play. Kc isn't needed - we can do Tr/Tr/Minion/Op/Masq or Tr/Tr/Urchin/Op/Masq or more complicated things like Tr/Tr/village/Scheme/Op/Margrave/Masq or Tr/Tr/Sch/village/Ghost Ship/Masq, etc. Even screwy things like Kc/Kc/Mountebank/Cutpurse/Masq will eventually work. Not even the masq is strictly needed to trash most of the opponent's deck and give him useless hands - you can go for Gov/Council Room to draw his deck and Ambassador/Possession/Top decking (e.g. Count) to leave the opponent with garbage on top and steal cards with Amb.
Other combos also have parts that can substitute:
Apothecary/Native village can swap out a Nv for cantrip/Bishop or Iw/Island options.
Counting House/Golem can have the same net effect if you ditch the Golem and use University to gain Inns and keep enough actions out to never reshuffle.
Golden decks can ditch Bishop and play village (any but Nv)/Grand Market x3/Nv/Mandarin/Prov (play through the Gm, play the village, top deck Prov with Mandarin, Nv away Prov) for example.
Chancellor/Stash can replace the Chancellor with Scav (as noted), but also Golem. Other options, like Cartographer or Warehouse can approximate the same mechanic.
Even Nv/Bridge can replace the Bridge with Highway/Market or HoP/cheap enablers.
Now virtually all of these subsitutions are inferior to the classic combo, often vastly so. But they still use the basic mechanisms to get the same net effect. Uniqueness of the cards is not really a requirement so much as a by-product.
The bigger thing about combos is that they
play weird with non-similar tempos. For instance, Chancellor/Stash has really weird tempo for what you buy. Few other options prioritize treasure to actions at 5, particularly for the first 5. Weirder you tend to prioritize a weak terminal silver at three over silver. Then there is the whole buy duchies whenever you can once you get enough Chancellors/Stashes, virtually nowhere else do you buy duchies before you buy your first province ... and yet most of your points will come from provinces.
Most games you fall into one of the other templates for when to buy say payload (e.g. coin), engine components/enabling cards, and VP. Combos through most of that out the window. You don't buy anything in a Pin game that isn't non-terminal or a Pin component once you have the cash to hit Kc (or 5 if you are using a cheaper alternative). Kc/Scheme setups get to buy duchies (or heck, estates) as soon as they have their few action cards and the Kc/Scheme on top looking for the other parts of the combo. Golden decks will most often have a good number of buy nothing (and even the odd do nothing with 7 coin) hands. Combos don't have set patterns of when to buy what and they are all over the board for what they care about (deck size, action density, copper count, etc.).
Looking at "does it play like a combo" tells me that things like Kc/Bridge definitely are combos, you just buy combo cards and anything that increases the odds of hitting it. Buying the odd province first is often worse than picking up another Kc. You love Cartographer but hate Warehouse (without live draw). Most treasure is actively bad. Likewise, University/Watchtower plays a lot like a combo - stock up on Unis, top deck your gains, draw them, and explode your deck quickly; lining up University and Watchtower can matter a LOT more than getting cash or even cheap, but useful actions you'd normally want in the engine (e.g. Fishing Village can actually be worth less than Oasis).