Clay has a section in his Pin Repair Guides about replacing power cords on EM pinball machines:

http://www.pinrepair.com/em/index3.htm

From the Pin Repair Guides:

"...For better safety, it's not a bad idea to replace the game's original two prong power cord with a grounded three prong cord. Again, the same technique can be used - buy an inexpensive three prong 15 foot extension cord at the Dollar store and cut off the receptacle end. Also again the new power cord should have a "ribbed" insulator wire which connects to the larger of the two power prongs. This is the "neutral" (white) wire. The "hot" (black or smooth wire or narrow prong) should connect to the line going to the power switch. The green ground wire should connect to the transformer's metal frame bolt. Optionally, additional grounds can then be run from the transformer's metal frame to other metal objects (like the lockdown bar and metal side rails and metal leg plates)."