Aye M4S4,
I suspect the sprocket has siezed onto the splines due to rust.
PlusGas A, Diesel or 3in1 penetrating oil any one of which will work, soak the splines and sprocket. Give the spocket a hitting with a 1lb hammer round the circumference of at the base of the teeth, with short, sharp, strokes, your try to vibrate and loosen the assembly, not drive it through the cases!

If this doesn't start to slacken the sprocket.

Get a blow torch or some heat and warm the sprocket around the shaft, NOT THE SHAFT, give it another hitting and some more diesel etc. Repeat. If still reluctant, leave diesel to soak for 24 hours, then apply more heat and vibration with iced water after.
If this still refuses to shift, disc cutter out and split the sprocket from both sides toward the shaft and finish off with a sharp cold chisel!

Desparate!!!!!
All the best and there maybe a load more suggestions to come from the bodgits!

Bill.