Stoneforge Mystic is to Umezawa's Jitte what Grim Lavamancer was to Cursed Scroll IMO, you definitely want between 1 and 4 in the deck and that depends on how far you want to push Umezawa's Jitte as the center piece of your mid-game. Personally I run 4 Mystic and 2 Jitte and I just steam roll past turn 4.
If you guys haven't tried Mother of Runes in the 1 drop spot you really should, she is such a counter/removal magnet and if she sticks, she gives you so much control over the board.
Regardless, I really like dropping Ape/Lion and white being a more central color than red, you really get to abuse Horizon Canopy at like 3 or even 4x and you don't need to curve to 3 for Thoctar or Knight.