Inspired by the Vintage Simulacrum Shops decks, here's my take of the deck in Legacy...
Simulacrum Shops
Lands (22)
4 Ancient Tomb
4 City of Traitors
4 Island
2 Otawara, Soaring City
4 Seat of the Synod
4 Urza's Saga
Creatures (12)
4 Sicarian Infiltrator
4 Phyrexian Metamorph
4 Thought Monitor
Spells (26)
4 Simulacrum Synthesizer
4 Nettlecyst
4 The One Ring
1 Shadowspear
1 Lavaspur Boots
1 AEther Spellbomb
4 Lotus Petal
4 Mox Diamond
3 Mox Opal
Sideboard (15)
4 Force of Will
3 Consign to Memory
2 Dismember
1 Haywire Mite
1 Vexing Bauble
1 Pithing Needle
1 Nihil Spellbomb
2 Unlicensed Hearse
The idea here is to generate explosive mana, draw a bunch of cards, and make a bunch of giant robots.
The deck also has Force of Will to glue it together, most likely with Force of Negation in the sideboard for unfair matchups.
I thought about Trinisphere (and Chalice of the Void) over Force of Will, but I think Force of Will is just going to be better at answering more of the format and this deck has plenty of card advantage and blue cards to easily support it. The deck also has enough other 3+ cost artifacts to properly support Synthesizer.
Sicarian is less broken in Legacy without Mishra's Workshop, but the synergy with Synthesizer is still insane. This deck is capable of making a pretty good amount of mana in the midgame though, where double squading Sicarian for 6U is definitely possible.
The floor of Sicarian is a 1/2 artifact creature for 2U that draws a card (and can be pitched to Force of Will), but the ceiling is that each additional 2 mana that you can squad it for is going to multi-trigger Synthesizer for each copy of Sicarian, for each copy of Synthesizer.
For example, a turn 1 Tomb + Diamond into Synthesizer, turn 2 Metamporph to copy Synthesizer and make a construct, turn 3 Sicarian for 4U to make two 1/2's and draw 2... creates 4 more constructs. The deck just goes nuts eventually.
Metamorph can copy Thought Monitor's to keep this deck chain drawing tons of cards, too. Metamorph can also copy opposing Bowmasters to kill their Bowmasters, although I think some number of Walking Ballista from out of the sideboard would also be a great answer to opposing Bowmasters.
Inventors' Fair and Fomori Vault are interesting lands the deck could maybe be running, in addition to the usual stuff like Otawara, Soaring City and Wasteland. Otawara probably needs to be in the 75, since the deck currently has zero ways of answering a resolved Ensnaring Bridge in the maindeck.
I thought about Urza, Lord High Artificer to help make big mana in the midgame, but I have no idea how to fit it, or if it's even necessary... it's probably just win-more, honestly.
Vintage Shops lists are running Nettlecyst, but I'm not sure how good that is in Legacy, especially without Mishra's Workshop, but maybe it's still strong in here? Being resilient to Dress Down seems pretty good, at least, and is a 3-drop to trigger Synthesizer.
Crucible of Worlds could be a strong card in the sideboard vs Wasteland matchups... and maybe even vs control matchups?
Portable Hole could be a good sideboard answer to Null Rod and Deafening Silence type of effects. Also hits Pithing Needle naming The One Ring, Bowmasters, Elvish Reclaimer, etc.
With Planar Nexus, maybe the deck should be running Urza's Tower instead for more explosive early game mana? Seems way less consistent than Urza's Workshop in the midgame though, especially without Expedition Map.
Speaking of Expedition Map, what are the best maindeck Saga targets? Pithing Needle? Vexing Bauble?
At any rate, this deck seems pretty resilient against most of the hate for it. Planar Nexus makes blue mana through Null Rod. The One Ring helps the deck recover from sweepers like Seeds of Innocence and Meltdown. Phyrexian Metamorph can deal with Orcish Bowmasters, while robots can attack Narset. Force of Will can also answer stuff.
Eldrazi Confluence seems like a cool sideboard option as well.
