Absolutely re-use fasteners that look good. I can't tell you how many times I have re-used caliper bolts, and they are far more important than engine mount nuts. The engine mount nuts really don't see a great deal of force, in my opinion. If they are not rounded off or very rusted, and assuming you dont over-torque them when you put them back on, they will be fine.

Sometimes when I am using a fastener for the nth time, I will tighten on the low side of the torque spec (if there is one) and use loctite to make sure it stays put.