Welcome to the board! You and I have the same problem; a 1918 South Bend without all the change gears, mine being a 13" x 60" bed.
Assuming you have SOME gears, and you probably have at least the ones on the tumble-reverse, figure out what Diametrical Pitch (DP) you require. (The definition of Diametrical Pitch is in almost every machinist's handbook I've ever browsed through.)
Measure the width of the gears, the diameter of the center hole, and the size of the keyway, where applicable. (Note that gears used only as "idlers" don't need a keyway.)
Look at the gear table to make up a list of what gears you need. (On my lathe, this is a brass plate on the left leg.) Note that some gears are used only on obscure threads; I don't need the gear that is used only to cut 30 tpi. (God only knows why that one is included in the table!)
So, now you have a list of what you want, and can send this out to whatever sources you may wish to try. You should keep a copy in your wallet, too - you never know when you might run across a tub full of gears at at Flea Market.
Regarding the drive, a previous owner of my lathe made up a pretty spiffy frame to hold up a jackshaft with a four-step flat belt sheeve about 10" above the spindle. The c-frame (face-mounted) motor is on the back of this frame. The frame clamps securely to the Vee-ways. It is a stout weldment made of 1/4" plate with a stiffening "web". First picture at
http://www.morrisconservatory.com/bbryan/jr/
(Ignore the erroneous id as a SBL Series O - it's a Model 34, traced via the serial number ledger at LeBlond to Sept 12th, 1918.)
My research indicates that most pre-1920 SBL's were painted black at the factory, with any color available if so ordered.
J.R.R.