How about a 604ccs engine in your 500 !

[Rusty's Uncle]

If any 500 owners are thinking about getting more power whilst retaining the original engine set up so the car looks totaly original as opposed to the 126 engine swop and all that goes along with it then I have got a kit of parts here. There is a bored out 499ccs crankcase to take 594ccs barrels, barrels, spacers and new oversize pistons that will give 604ccs. I have done the same with a 499 to 652ccs engine and it means that you can accelerate up hills rather than keep looking in the rear view mirror plus the "concours police" do not know your classic is not original 

[grubscrew]

 I had a 594 in my "Lusso", not a bad little motor. I would`nt mind trying a stock standard 499 just to see. It cant be any slower than my little isetta of around 300ccs !

[Rusty's Uncle]

This engine is now in the latter stages of assembly and is now sold but I will be putting a few items in the for sale section as I have just had some 500 barrels rebored for a new set of pistons I had and I have put in some spare 500 & 126 crankshafts for regrinding and a couple will be available soon