If the spell you cast has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

You choose which spells to cast (if any) as Kylox, Visionary Inventor's last ability resolves. If you choose to cast any spells, you do so as part of the resolution of that ability. You can't wait to cast them later in the turn. Timing restrictions based on the cards' types are ignored.

Use the power of the sacrificed creatures as they last existed on the battlefield to determine the value of X.

If you cast a spell "without paying its mana cost", you can't choose to cast it for any alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, those must be paid to cast the spell.