I don't think this deck is going to be competitive by any means, but it's definitely fun to try to win using the alternate win condition from Darksteel Reactor by proliferating as much as you can onto it.  The sideboard mainly just has utility artifacts that I could think of that might would be good in certain situations, as well as the 4th copy of Darksteel Reactor that you can fetch with Karn.  Also, proliferating onto Nissa, Who Shakes the World and then using her ultimate with an Evolution Sage or two on the battlefiend is typically game over when you have a few counters on a reactor already.  Veil of Summer may be worth playing in this deck to avoid having it stolen with Agent of Treachery or Hostage Taker, sacrificed to Angrath's Rampage, or turned into a 3/3 Elk... because Oko, Thief of Crowns.  (Small update: Veil of Summer has been suspended, so it is no longer an option to defend againse Angrath's Rampage, Agent, etc.  At least Oko is also suspended, so you don't have to worry about it becoming an Elk.)

Update:  My first version of this deck had too many non-forest lands (especially plains).  I removed 3 plains, 3 Blossoming Sands and 1 Karn's Bastion, and replaced them with basic forests, but playing two Karn's Bastion is probably enough if you want to replace it with another forest.  Using Gingerbread Cabin just feels pretty bad since it would still enter tapped from Nissa's -8 ability, keeping you from getting the food token for the extra life.  Additionally, it may be worth having some low cost artifact creatures such as Ornithopter and Clockwork Servant for additional card draw with Karn while The Great Henge is out for you.  Pattern Matcher may even be worth playing in the sideboard to get another Evolution Sage on the battlefield before you use Nissa's -8.

Update 2:  I made changes to the sideboard to include 2 Ornithopters and 2 Clockwork Servants for Karn's -2, and removed Treasure Map // Treasure Cove, Mana Geode, Chromatic Lantern, and Firemind's Vessel to make room for them since they seemed fairly unnecessary.  Adding Veil of Summer to the deck is starting to seem somewhat necessary, so it may replace Elvish Rejuvenator.

Update 3:  The white proliferate cards felt unnecessary, leaving the deck where it was trying too hard to do one thing, and it wasn't doing it well enough.  While the main wincon is to proliferate onto Darksteel Reactor as much as you can, going mono green feels more consistent, and benefits Nissa's -5 as well.  Also, Once Upon a Time has been suspended in Historic, so I have replaced it with (the vastly inferior) Bond of Flourishing as an alternative to try to dig for Darksteel Reactor, or another permanent you may need in the early game.  (If Veil of Summer wasn't also suspended, I would probably just slot that in for OUAT instead.)