D2 5U pony motor clutch stuck engaged

Pinion clutch is likely gummed up. Drain the oil from the compartment and fill/flush with kerosene or diesel. Might also need to loosen the clutch adjustment and get it to slip a bit before retightening it later. The pinion clutch does not need to be very tight to pull the diesel over, but it sounds as if that one is already loose. Refill with same oil as diesel and pony motor, or use ATF which causes less drag in the oil clutch especially in cold weather.

Could be something inside the clutch housing that's come loose if the clutch lever really just flops back and forth. Most later tractors have a small cover you can take off that gives some access to the clutch linkage and adjutment collar.

The pinion lever is supposed to just flop back and forth once the pinion is latched in. If you just want to get it loose from the main engine, you can remove the small cover over the pinion (near the left brake pedal on the clutch housing) and punch one of the latch adjustment screws with a screwdriver or something and get them to release. You will still be starting the pony with the drag of the pinion on it, but some start right up like that. A well worn pony or one that has other problems won't be easy to start with the pinion turning too.

I once had an exhaust valve break in the pony motor just after I had enguaged the pinion. The pony still ran but didn't have enough power to turn over the diesel. Long story short, I had to fix the valve. Once the valve was replaced I used the pony to start the diesel (pinion enguaged the whole time during the valve replacement.) It too was turning the fan with every pull on the pony.....
but as soon as the pony started the fan no longer spun. I just simply went threw
the regular start-up procedure. The diesel kicked out the pinion once it started and have not had that problem again.

If you are as yet unable to unstick the clutch pinion, your problem will unfortunately require the intervention of a Lamborghini mechanic. Your association with all things Enzo is causing you to channel his hatred for tractors and interfering with your ability to perform the repair. A mechanic versed in all things Feruccio will be able to reverse the curse and give your D2 the same loving attention once given to tractors from Modena.