Kymco super 8 problem

Hi, I have a kymco super 8 50 2t, I recently cracked the piston cylinder head and had to replace it with a new one. I ordered a new one that said it was compatible with my bike however it’s a windowed piston ( my old one wasn’t) the bike seemed to run okay however when I took it back onto the road it couldn’t hit 40 which was my original speed and only goes 30-32 (it hasn’t lost power as it goes 30 up hills)
It sort of bogs and refuses to go past 32 does anyone know what the problem could be I know it’s a long post just wanted to give the details thanks