250pF is an odd value. Since the capacitor is simply an input coupling capacitor then a 220pf or 270pF capacitor at almost any voltage will be fine.
A regen radio was designed to work when there were hardly any radio stations around so it was not overloaded. Today a regen radio will be seriously overloaded by all the very strong local radio stations unless you live far from civilization or use a short antenna wire.
Note that it is an AM radio. AM radio sounds awful.