So to get back to this thread and provide some information and pictures for those who might face similar problems...
I mentioned in my last post that I will leave the ATV standing for a day or two and observe any ingress of 2 stroke oil into the counterbalance casing. I have done that and opened up the drain bolt on the counterbalance casing last night to find clean 2 stroke oil in this casing once again... Remember at this point the ATV was just standing for 2 days, the counterbalance oil was previously drained completely and the motor was not started from the previous drain.
As mentioned by latebird previously:
It would be unusual for an oil pump to leak around the drive shaft, but the pump is driven by the counter balance and any oil that might leak around the drive shaft would end up in the counter balance drive chamber
The evidence of 2 stroke oil once again just confirmed to me that the 2 stroke oil must be leaking from my oil pump into the counterbalance. The leak must be so severe, because it's leaking via gravity (2 stroke tank higher) into the counterbalance just from standing for 2 days.
I then proceeded to remove the oil pump and this is exactly what I found (see photo)...
I left the oil pump, with supply line connected, hanging for about 5min and when I got back to the garage there was already a small blob of oil on the newspaper (see photo).
I do remember from re-doing the entire motor that I also spend a good hour or two on the oil pump by completely disassembling/cleaning it. Due to the motor being in the condition that I originally found it, the drive shaft of the pump was rusted and pitted quiet a bit (see photo). I tried to service it to the best of my capabilities, without realizing that it might leak 2 stroke past the shaft and oil seal.
For now I can consider replacing the oil seal (bottom of pump) around the shaft, but I don't think this will resolve my problem due to the heavy pitting on the shaft. I think it will still leak to a certain extent.
I now basically have three options:
1. Have the existing shaft fixed up by an engineering shop and install a new oil seal in the pump
2. Try to obtain a 2nd hand pump somewhere (Not knowing the condition of this spare)
3. Remove/blank of the oil pump and run a Pre-Mix. I will need to search this Forum to find relevant advice and information to see if this is advisable and what needs to be done.
I hope that this information might be helpful to someone out there. Next time I will definitely test the pump prior to installing it into the motor and consider replacing the oil seal on the drive shaft of the oil pump.