What a nice tractor! I get a litle nostalgic when i hear it.
For the PTO there is two levers one for engagiging at the steering column, wich sometimes can take quite log time to engage thus it is hydraulic, up to a minute or moore when freezing cold! Mayby check filter for manouver cirquit. The other lever is on the left side of chair base. It is for choosing dependent(if available), neutral and independent. Check if it is in neutral..
The pto tap is exchangeable. The one not in use is stored in a vault on the right side of gearbox, covered with a romboid tin cover secured with twoo screws. You see it inside right footstep. If you have a 21 splines there and exchange them, the placing of 6 splines in vault makes it shifting gear so the pto turns in 1000 rpm.