I had an '88 Force 125 on my first boat, but I never fiddled with the tach, which BTW, was this big, square, color-scale thing that looked more like a toy. That's not really much help, but I think you already know you're probably buying a new tach. Here's something that might help...
http://www.surplusunlimited.com
Prices are overall lower than the big catalogs and sometimes they have closeout stuff for even cheaper. And of course, there's always eBay
http://cgi.ebay.com/ebaymotors/Faria...QQcmdZViewItem