I believe you'd have to change the outer slider collars (the part that the fork rides on) and ALL the gears which take a syncro ring, so the input shaft, 1st, 2nd, and 3rd gear.
MAKE SURE the gears are compatible with the cluster. Not only ratio, but helix. I have no idea what liberty uses, they may not be a standard helix angle. So you may be looking at a new cluster, too.
Or, just learn to double clutch.
Driving a model A:
I don't know what all the crap on the shifter is, he must have a two speed rear
[ame="http://www.youtube.com/watch?v=Vlf09huxvHI"]Double Clutch Ur Model A.wmv - YouTube[/ame]