10" to 8" VBB

I just installed a PX125 engine on my VBB. I will keep the 8 inch as this keeps the original look of the bike. I'll use the coversion kit  : http://www.sip-scootershop.com/main/base/Details.aspx?ProductNumber=50015160&_language=en.  To avoid High rev's I need to change the ratio, but not sure how.  Is it sufficient to to replace the clutch with an additional tooth ot best primary as well, what is the best combination?

Thanks a lot for your advice