Replacing Boiler Supply and Return lines

OldHouseGuy

About 10 years ago we purchased a relatively small home that was built in 1922 that had a hot water gravity system installed.   I have since had the boiler in the basement replaced with a Buderus gas fired boiler and the installer added a Grundfos circulator.  All of the cast iron radiators and the original iron pipe suppy and return lines are as they were originally installed.   We are wanting to finish our basement as our budget allows and part of this process will be replacing the low hanging (head banging) gravity lines with either copper or perhaps Pex with an oxygen barrier (?).   The primary supply and return lines are 2" iron pipe and the stub ups to the radiators are between 1/2" and 1" depending on the size of the radiator above.  There are currently 11 radiators that heat the house (8 on the first floor and 3 on the upper story).  Each radiator has its own shut-off valve.  The boiler has 1 1/4" supply and return lines that are adapted up to the 2" iron pipe.  I assume that since the system now includes a circulator, I could reduce the main supply sizing down and also would like to consider adding a couple of zone valves to only heat the upstairs or downstairs as required. I would truly appreciate any guidance on the replacement pipe sizing.  Thank you!