i don't think there are parts kits, but should be able to get the o-rings you need from
airgun designs or
tunamart.
which on/off asa do you have? also, you don't have to screw your tank all the way onto the asa (in fact, i don't think i ever do). once it's pressurized the tank will be solid and won't move. leaking through the cap...is that the part you twist? or do you mean one of the plugs in the side of the asa? if it's the part you twist (knob), you'll need to service your asa. that could mean just cleaning it, but most likely the o-rings inside are done and need to be replaced.