OK that info helps.
Notice that the "leaky nut" in question is on a stud that protrudes from the transaxle case, while it's partner securing the transaxle case to the engine crossmember is a capscrew? This was done on purpose by the engineers because the threaded hole for the stud has to be a controlled depth; it cannot be too deep or else the fastener in that hole will break thru into the case and in extreme situations possibly impinge upon the shift operating rod. They could have just used a short capscrew, but they knew that out in the real world not every mechanic is the best at putting the exact correct capscrew back into exactly the correct hole.
I recall Steve Hoelscher, a longtime member of this forum and undisputed King of X1/9 transaxles, relating this explanation many years ago.
It's possible that when the engine and trans were removed and then replaced, someone did not know the limitation above and ran the stud too deep into the case and created a direct path thru which transaxle oil can seep.
You can conduct an easy experiment to see if this is the problem. Place a catch pan under the suspected area, double-nut the stud, and then unwind the stud out of the transaxle. If the transaxle oil starts flowing out like a drain, then you have discovered the issue. The solution would be to let it drain, clean the area and the threaded hole as best you can, then apply a good teflon sealer or teflon tape to the stud threads to seal it up.