Recommended sparkplug is not working Stage 6 streetrace 70cc

Hi, I recently fitted a 70cc stretrace kit to a Yamaha Aerox, I use the recommended BR9ES plug but it seems to hit the top of the piston and prevent it from firing. Have tested with an old B7HS plug which makes it start and roll forward but it is short threaded. Any suggestions? I have fitted all the included gaskets and measured the squish at 0.7-0.8mm

 

Run with the shorter plug, won't do it any harm.
But make sure the longer one hasn't holed the piston crown

 

Won't I loose performance? It will not be positioned right in the expansion chamber

 

Not that you'll notice. Plus it's not so close as to cause a firing hole in the piston crown.
A lot of people make the mistake of running the plug too close to the piston when calculating the squish. If you get too close, the spark on the plug electrode erodes the very top point of the piston causing a hole in it.
Basically, it's spark erosion. And happens very quickly on aluminium.