This is an excellent saw. I've had mine since 1969. Re the Key: remove the handle cover (two screws - I think), remove the locking mechanism - Now, it's just a metal counter weight that slides on a "pin". PUll it out, replace the cover and the saw will work. Remember, now you have no locking device and the trigger is totally active. All the "key" does is move the counter weight which allows the trigger to "pull".
Hope this works. PS I actually have two of these saws. My son has the original, and I purchased a second one. I still use it on an active basis, It's October 2018! I have had the motor rewound, but that's all.