Hi there, I've acquired an old but beautiful

AHD Exocet LE SCS. I believe 1996 model, 270x53 cm.
Would any one know what size fin would suit it best? It came with a concrete wave
slalom like fin, but I believe it restricts its speed potential. Ive only managed to get 30 knots out of it. it accelerates amazing but then signs out.
Does anyone also knows if at the time was intended for flat water or can it handle chop too?
Finally, it mentions that it will support 80kgs, would that be the equivalent of 80 liters these days?
Thank you in advance for your help.