How can I safely put a plug on an old furnace motor that has just two wires
You shouldn't just attach a plug directly to the leads coming off a motor. 

Mount the motor in or to the rest of the machine, and assuming it's metal, this establishes a grounding bond between them. You can then mount a small junction box, and terminate the wires from the motor in there. Be sure there is continuity from the motor housing to the junction box -- it may be necessary to add a ground wire for this purpose, especially if the motor itself has a ground lug. Grounding all the metal parts of the machine is important for safety. If there is a short, without a ground, anyone that touches an energized metal part will be electrocuted. With a ground, the electricity will follow the path of least resistance (the ground conductor, instead of the human touching it) and in most cases, enough current will flow that within milliseconds the upstream fuse/breaker will trip and the power will shut off.

To add a plug, use some SJO-type wire (that has two conductors plus ground) and connect the ground to the junction box (thus grounding the junction box, housing, and motor housing), and make the connection to hot/neutral in the junction box as well. Your connections should be either wire nuts or a terminal strip of sorts. Be sure to use a strain-relief connector for the SJO. It's also not very hard to find pre-fabricated lengths of SJO with a plug already moulded on the end -- just ensure it is rated to carry the current the motor draws.