the language of my ancestors, as picked up from my father, raised a few giggles - kartofle, pióro, pójdę do ustępu, itd!
'Kartofle' is OK, but the name seems indeed to be replaced more and more often with 'ziemniaki' these days.
'Pióro' is OK, but you won't see it much in use these days. The thing which you use for that purpose is referred to as 'długopis' (most often) or 'cienkopis' (colloquially 'pisak').
'Pójdę do ustępu' is correct only if you mean going to a dry toilet which is situated outside the house (quite rare these days) by this.