Used glass is never a good deal...
It often is damaged by the removal process...and it cracks eventually from small knicks in the periphery...
New glass for a neon is pretty cheep...I would guess about $100...but that's my garage price...you would have to find someone to sell you one at a decent price...look in the yellow pages...
Replacing a windshield is not that difficult...Just make sure to order the upper trim with the windshield, if I remember well they have a rubber trim that sits in the urethane that will be scrapped when you remove the windshield.
You simply cut around the glass to cut the urethane bead. Then you glue the new one in with a windshield urethane. Try dow's U-416 which is a one part primerless urethane that is dispensed in a regular caulking gun...Make sure to look at the tip of the urethane tube and cut it like the little drawing. This special cutting method creates a little triangle shape that leaves a pointed trace of urethane on which you will lay the new glass.
Don't remove all the urethane from the window frame, the new urethane is made to stick to the old urethane...
Make sure to clean the glass and the window frame before re-installing...
My 2 cents worth...
Serge