Katana Owners Club U.K.

Bigger rear sprocket: better acceleration, less speed.

I did this and wrote an article about it for "The Pride: Summer 2015". The link to the PDF in the forum appears to be broken, but here is the info in a PDF for you to look at:
Although I went for the 49 tooth I think it would probably be better with the 48 (or even 47 for a more relaxed ride long distance ride). As long as clearances are OK, 46 will work but you will lose acceleration and possibly depending on the power of the motor not be able to pull top end. Cruising revs will be lower though. The GearingCommander website (link in PDF) is good to show (in thoery) the speed at certain revs with different sprockets.

but a common rule of thumb is that 1 tooth bigger on the front is a very rough equivalent to 2 off the rear, so, if you fit a one tooth bigger front and a 4 tooth smaller rear, the difference wont be that apparent.
although its also worth saying that smaller sprockets will have a slightly elevated wear effect on chains, so the smaller they go, the more 'work' the chain has to do to get round it, so in the long run, bigger front sprockets are less wearing on the chain, but this is all a bit academic really, its all very slight effects. what really matters is to make the bike feel less frantic when youre riding it.