I'm looking for a narrow kerf bar and chain for milling with my Stihl 064, your recommendation please.

Stihl don't manufacture anything specifically for milling. However, do keep some ripping chain loops which is used for planking and may be suitable. Chain loop RMX 3/8 1.6 - 3653 000 0098. 30” Chain bar 3/8 1.6 – 3003 000 6041 Chain loop RMX 3/8 1.6 – 3653 000 0114. 36” Chain bar 3/8 1.6 – 3003 000 6053