Been rooting around in the spark plug info and found the following: The Autolite 386, 86 and the NGK XR4 are all resister type plug so I'd cross them off the list even though they fit.

For the 14mm Cat #8F5899 which is a Champion UJ-12 the best candidate replacement is the NGK-B2 or an Autolite A11

For the 18MM Cat #2A3643 which is a Champion D-16 the best candidate replacement is the NGK AB6 or Autolite B8 or BT8