Hey, 88kdx. It's nice to see that someone else has the same bike.
Check out http://www.electrexusa.com/electrex_speedometers.html , I personally havn't used that product, but it looks like a good buy. I'm not sure if it will fit an '88. It's worth a look though. Someone else might have some experience with it.
I know that they have things like this for bicycles, and they're alot cheaper to(you can get them for around 35$ Canadian). If I were you I would phone up a bicycle shop and ask if it could somehow be mounted.
-Don