Here is whats in my kart tool box . 3910x in the engines .
I would use any one of them , all honesty the 64 was pretty good before the 3910x came in play .
your choice
4-autolite 64
3- ngk bpr6es
2-denso Q20r-zu11 -----denso w29es-zu (cold)
one b9es its pretty cold
and a ngk#8 in the snowblower.
a 411 a-j19 flathead plugs