Welcome Adolfo,
'Matching numbers' is generally not considered to be important in the Pagoda world. Not having a 230 engine but, rather, a 250 engine, may detract future buyers just a bit but, if properly installed, should not make a noticeable difference in your enjoyment of the car. Replacement 230 engines are difficult to come by. If the seller had represented to you that the car was original or 'matching numbers', then you can see if you can negotiate a price reduction. But, if the car is in good condition (no rust) then I wouls not walk away just because of the engine not being original. Good luck with deciding,