oversized pistons? what are you talking about? you can just use 4age pistons BT ST 16v it doesn't matter i used over sized pistons because my bores where fucked and it needed a bore and hone but if your asking where to get them any car related place can order them in i work for an engine reco company so i get them cheap
and yes BT 20v oil pumps suck the hydro's shit themselves its better to run a silvertop or a 7a im pretty sure a 7a pump can pump more fluid than a 4a pump as the gears are thicker i can confirm this at a later date i have spare of both laying around so ill mic them up see if there is any difference