If you remember what the key looked like, you can make one yourself This will be easy if you are somewhat crafty. There is another way. If you contact the treadmill company, they can send you a replacement. Each company has a different shape or type of key. Call the company and ask for a new one, or use a piece of thick plastic to make a new key. Body by jake doesnt make a treadmill anymore so you will have to refer to your paperwork. I have done some research, but without knowing the exact name of the equipment, I can really give anymore info.
The best and probably easiest way to fix your problem is to remove the panel that covers the area where you would put the key in. You connect the wires that are of the same color. You see; When you put the key in (the key is only a piece of plastic) the key will only press on a panel or pin that will connect the circuit that will turn the machine on. (Caution!- you are doing this at your own risk) the only reason I know how to do this is because I am an electrician). By taking away the safety feature on this treadmill, you void the warranty and possibly may present danger to yourself. As I said before, this is the easiest way, as there is probably no way that you could order a replacement and it would take a long time and a lot of luck to do so. SO, all you do is unplug the machine, take off the panel, connect the (alike) colored wires and make sure to use electrical tape or the appropriate twist connectors to avoid electrical shorts(and possibly fires).I hope this helps.