In 1950 the ASAE (American Society of Agricultural Engineers)introduced the larger 1-3/8" diameter spline PTO shaft and has been the industry standard ever since. They don't make or sell the old 1-1/8" PTO shaft anymore. There are low cost solutions to use if you have the OEM PTO shaft. The cheapest is a simple sleeve that slips over the shaft and pins with a roll pin -see pic. For a few bucks more but a lot cheaper than replacing the entire PTO shaft is an ORC (Over Running Coupler) that attaches to old PTO 1-1/8" shaft and has the 1-3/8 shaft for use.
PTO ADAPTOR SLEEVE: