Jet as you probably know are a badge brand. They don't make anything themselves but have manufactures add paint colors and badging with there brand. For a Taiwan 1981 era machine, look for any other 12" x 36" lathe that has the same approximate build date and exterior features. Any manuals you find if you get lucky enough to do so will be the same. What helps is the mainland China machines were a lot less common then, so same size/ same layout, then it's highly likely it was built by the same manufacturer. If you still have the motor mounting plate, measure it's bolt slot and the pitch spacing and start cross referencing that against the published dimensions the larger motor manufacturers list. If the belt is still there it will give you the approximate motor C/L minus a couple of inches for the pulley diameter. It will get you in the rough ball park at least. There are at least 3 motor standards in use today, Imperial, Metric and a separate Asian based standard that doesn't match up with either of the first two. Impossible to say if yours was or wasn't built using it.