Taking a 1/16 out of the key can be a problem if the hub (drive) has a hundred thou depth to the keyway as you would remove around 62 thou leaving a 38 though depth, that's less than 1 mm of key engagement.
Sometimes I have hubs that you can't remove 1/16 as they are not very thick to start with and some casting shifts leaves the machining thin on one side.
I wouldn't have a problem putting a second hand crank in a new briggs motor as it should measure up fine and is not much different than a reco motor.
As Ironbark suggested there are lots of youtube videos dismantling and assembling Briggs motors.
Probably the easiest thing for you to do is to change the coupling and shorten the shaft as others have suggested .
I would also try to free up the original motor ,I've had a lot of Briggs seize on the big end con rod from running out of oil , put some WD40 in the crank case or a fair amount of oil and make sure the lubricating fluid reaches the rod by tipping the motor over then use a big shifter on the crank to turn the motor , you will need a key in the crank so the shifter doesn't slip and this can sometimes damage the key if it's really tight to free up ,but you can use an old key or buy a new key.
Once the motor is turning remove the spark plug and make sure the motor turns easily this can take a while , sometimes I use a drill to spin the motor over.
I've had a lot of Briggs motors that I've done this too and they have lasted many years afterwards ,I've also know sometimes they don't last very long if the conrod lets go and some that people remove the conrod and linish the alloy off the crank big end, those linished seem to have too much clearance and you end up with knocking noises and don't last as long.
