I am aware that if you´re dead set on an original Jensen #15BL, this posting won´t be much good, but if not...
Perhaps you could lure Bruce "Saint Paul Steam" to upgrade a standard genny with ball bearings for you.
He has done this with great success on several during the years 
I am not dead set on the 15BL and that’s why I worded it as such. I would be more than happy with the SPS versions.