Rasta,
Here are a few links to look through.
http://www.carbparts.com/keihin/kdirt.htm http://www.sudco.com/ The clymer I have says the '87 kx250 stock is a Mikuni VM38ss
(pg.334 Table 3) I don't have the specs on an '86
The 1988 came with a PWK 39
A few other options you might consider is a PWK 38 airstryker
or standard PWK 38
A new carb is probably gonna' run $200. ish
Re-building yours or another used one there are a few questions you might want to think about.
The sum of the parts may sometimes cost as much or more of a replacement.
Ebay is another option. (but has it's risks)
I guess it depends how much you can spend,how modified is the engine and what type of riding you mostly do, MX,Dunes, Woods etc.
Tuck\o/