For those looking. I answered this over on another site.
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I had a lot of fun designing that class.
Scavenge Field Repair is an "improved" Field Repair Feat (ultimate combat). "you can repair that item with no raw material cost and no penalty on your Craft skill check for using improvised tools. If you spend a day, the item regains 1 hit point plus one-quarter of its original hit points."
So it takes a day, and repairs x hit points as per the make whole ability.
IE: Joe is a Warsmith lvl 4 with 4 ranks in the proper craft skill. He is going to use his Scavenge Field Repair to fix the heavy siege engine with little to no tools available (no tool kit). Since it is is basically a modified Field Repair feat, he spends the day repairing it. He makes the Craft DC check (20) but instead of the engine receiving 1 + 1/4th the original hp, it receives 4d6 hp instead.
Say the engine has total hp of 100. Instead of receiving 1+25 hp, it receives 4d6 instead.
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1. So it takes a day to repair the object using this ability. (It does seem to have bad wording, but it uses the Field Repair Rules).
2. As per number 1, it would take a day to repair. But the trade off is that it doesn't cost ANY gold to repair an item.
I think what might be acceptable (what I would accept), if you talk with your GM, if your Warsmith would be able steal the "Reforged" ability from the Soul Forger (magus archetype) as a Design. But you must be at 8th or 10th level to select it. I might have to add it to Liber Xpansion this weekend.
Did I answer the question for you?
EDIT: So assume the siege engine takes 8 hours to repair: if you take the Quick Fix design, it cuts it down to 4. With Repair Master Design, you add + Xd6 to your repair roll.