| A 583 with a bore kit still only makes 51 to 52 bhp not exactly competitive with a 620i, then there is the better throttle response from the injected bikes plus a fair bit higher rev range and on top of that some 620s have 6 gears.
I always though a good option for 2006 would to allow class A bikes with a 583 based engine taken out to 675 and allow some head and cam work but possibly still running on standard CV carbs be allowed to run against the 620s.
I 'm buying Glyns 675 kit off him for my old engine and if I have the time and spare cash I will probably have a look at doing something with the heads to see what a bit of porting and flowing does for them. | 
skidlids
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