Fixes are not simple! ZF speedos are fork-and-tongue driven, and over a long period of time an unhardened tongue or fork will crack from age and vibration. Withthe right angle adapter, two things occur: the adapter must be installed with the double-end nut threaded UP on it as far as possible, then screwed DOWN onto the threaded part of the ZF as far as possible. Note that only moderate tightening is needed. Lube leaks are fixed by replacing an o-ring, not by over-tightening the double-nut. This installation method maximizes contact between the tongue and the fork and minimizes the leverage each has on the other. This only applies to the Veglia-Borlotti right angle adapter with spiral-bevel gears. The earlier VDO adapter for pushbuttons & Mangustas use right-angle gears and a more complicated method of construction.
Assembling any other way causes leverage on the fork & tongue parts; continued hammering from vibration during driving will crack the drive parts. Second, over time (now 50 yrs!) the OEM grease inside the sealed right angle adapter dries out and becomes brick-like. This causes more friction inside the adapter which can help crack the drive parts. I've had a few that were fully seized. Sometimes, overnight soaking in thin oil will soften hard grease. I've fixed a number of broken right angle drives, but the part inside the ZF is a separate 4" long shaft with an integral gear machined on it. Replacement is the only viable fix there.
And be careful removing the little shaft & sleeve from the ZF: if you lose your grip while removing that 10mm bolt, the shaft may slide down inside the cases, forcing full disassembly to retrieve it. Driving with the little shaft loose inside WILL crack the aluminum ZF case, and TIG welding following compete disassembly is the only fix for cracked cases. It may also chip a transmission gear $$$$. Note both the internal shaft and right angle adapter have been out-of-stock as NOS since about 1990 unless you get lucky somewhere. The last NOS I saw for sale was from Roland Jaeckel in Germany some 20 years ago. Good used parts are virtually your only choice AFAIK. Also note there are FOUR different right angle adapters- two styles in two different gear ratios, so be cautious buying a replacement without comparing what you know fits your car (as usual). See Bulletin 1, Article 6 of the TSBs. Good luck-