Sorry, backwards brain fart. But the theories hold true.
Most of the time people are asking if the 87-92 will fit the 85/86, not the other way around.
Now that I am thinking straight, an 87 with the mods that you stated is definately worth $1000

There are a couple motors that will fit your quad on Ebay right now

 

A zilla 500 on a 250 would be sick!!! Its like dropping a 5.0 v-8 into a mazda miata! I would love to drive a quad like that[img]i/expressions/face-icon-small-happy.gif[/img]