I noticed that someone mentioned using the B8ES plug. DONT!!!!! There have been several reports of the electrode breaking off at the weld and dropping in the motor. The ES plug is not that great with vibration. Use the EG plug. If you do a search there was a NGK apps engineer on this list that posted this info as well.
As for 40:1 not being good enough for small bores...I'd disagree but have to qualify that statement. If you use a good quality oil like Bel-Ray Si7 or Motorex Cross power 2T you'll be fine at 40:1. We use 40:1 exclusifley in our Cobra 50's, KTm 65's and yamaha yz85's. With riders that ride hard getting top 10's at nationals including one national champ.