I think other threads have suggested too small a vent hole can allow oil to seep out, as it can froth a bit on being pumped into the restricted space at the top of the oil tank, perhaps open up the hole a bit on the inside of the cap.
I used a replica (fibreglass) burgess type oil filter on my huntmaster, but never bothered too much with what went inside, if you use oil perhaps use a tacky type (I think there is an oil made for use with scrambles type foam filters). The positioning of the inlet at the rear of the filter will greatly reduce intake of muck, especially in a damp climate. You may need to occasionally wash out any muck accummulated inside the filter.
I believe Roy Price, who advertises in OBM and other publications, reconditions ATD units, although they are not that cheap. I have repaired them in the past by riviting on new fibre gearwheels (sold on Ebay), and replacing the self extracting centre nut, but if the extraction thread in the body of the ATD unit is damaged, it can be nearly impossible to remove it from the magneto shaft.
I have run a huntmaster in both manual and auto advance format, I prefered the auto setup for better control over low speed running and tractability (less fiddling with handlebar controls at low speed/when manouvering). If moving to manual advance, you may need a new end and cam ring for your magneto (as well as a cable and handlebar control), or you will be stuck with full advance all the time, this can be lived with (as PeterW's reply above) but is not as good as having retard at low revs.
Also be aware there are different advance options for ATD units, you would need one with around 17 degrees.