I suspect that you might be in for some disappointment - it's not impossible that someone has the appropriate mounting bits for the bike computer they used 20 years ago stashed away in a box of [strike]junk[/strike]miscellaneous parts, but the odds of them being listed on Ebay are pretty much nil.

The best thing I can think to do is to measure the base of the computer (the part that slides into the mounting bracket) and post those dimensions on this forum along with a picture, maybe somebody here has the compatible part and will send it to you for the cost of shipping. I would post that information in this thread since it's already started, and in the "ISO and For Trade" thread that's stickied in C&V. Good luck.