During the belt replacement procedure, you disconnected or unplugged (or should have) the 2 wire harness that leads from the chassis to the PTO clutch.
Can you confirm that the connector halves are once again fully seated into a secure "hand shake", meaning that the locking tab and slot are fully engaged?
If the connector halves are back together properly, your PTO switch may have gone south due to age.
And there is also a chance that the PTO clutch itself may be defective.
It also might be that the clutch is in need of having its air gap adjusted (if it is the adjustable type) but I'm not leaning toward that very much because typically the need for reducing the air gap is shown when the PTO clutch DOES NOT engage at all.
One way to determine if the PTO switch is defective is to test for battery voltage on the red wire of the PTO clutch circuit with the PTO switch engaged and key switch in the run position.
Another test is to run a jumper wire from the PTO clutch harness red wire to a 12 volts source to see if the clutch operates when the PTO switch is completely bypassed.
I am not really enthusiastic to recommend the bypass method to amateurs and DIYers because I have no way of knowing their specific skill level, and there are a few ways in which the bypass method of running a hot jumper wire can go very wrong.
But if your skill level is reasonably high, you can accomplish a bypass test without mishap. The biggest concern I have in the bypass type of test is that the actual connection of the jumper wire to the 12 volts source be made somewhere NOT ON THE BATTERY, and that the connection to the 12 volts source be made LAST. That way you don't have a "hot wire" that will short if it touches a grounded surface.

Thanks will try that and get back