The only two option for the rear tiers in Pirelli Trefeo R are 315,30,20 and 335,30,20 while the original size is 325,30,20. Not sure which way to go 315 or 335 for the width?
Original 325/30-20 = ((325*.3) x 2)/25.4 + 20 = 27.68 dia
315/30-20 =((315*.3) x 2)/25.4 + 20 = 27.44 dia
335/30-20 =((335*.3) x 2)/25.4 + 20 = 27.91 dia
One is 6mm taller (3mm on radius) & 10mm wider (5mm from center out) and the other is 6mm shorter and 10mm narrower.
Safest bet would be to go to the 315/30 - they'll definitely fit.... but obviously a little narrower than stock.
I would assume the 335/30 would work too but you take a small risk in rubbing?