I thought the 45 mm is the best thing for a 851/888. On ebay germany there is a 50mm collector, but that is for a 926! You can put it on your 851/888, but you won't get that much advantages.
Hi, the size as in 45/50mm is more to do with the lift,timeing and size of the exhaust valves, than the capacity of the motor. The 926 would be running bigger valves and latter full race cams, i think. Steve Robins s.w.