Cagiva North America did not sell any 1987 model year F1b's in the USA. The Laguna Seca was an 1987, but not the F1b. The tenth digit of your VIN is the model year, a G is an '86, an H is an '87 and a J is an '88.
If your bike is an 1986 F1b, then it should have a G for the tenth digit, and would have Vitaloni mirrors that are the Sebring type with a special Ducati mount, with 75mm mounting hole spacing.
http://www.talbotco.com/vitaloni_pics/s ... lack_2.JPGLoudbike had a bunch of these for sale, but they may have sold out. You might see if they have any left. These used no metal support plate on the inside, only some small washers.
If your bike has a J for the tenth digit, then it is a 1988 F1s, not an F1b, and uses mirrors similar or the same as those that you already have and are available from Bevel Heaven, and Loudbike. These used a metal support plate on the inside of the fairing.
https://store.bevelheaven.com/body-dres ... or-750-f1/http://www.loudbikestore.com/067099060- ... 14845.htmlYou could contact either of the two Steve's at Bevel Heaven or Loudbike and have them measure the mirrors that they have to see if they are closer to your 66mm mounting. If not, then it's time to modify either the mirrors or the fairing.