there are alot of options, you could replace the dash entirely(
www.motogadget.com or
www.kosonorthamerica.com ) or you could go the DP route and get yourself a no longer produced DP gauge cluster that has a tach, i went that route, but the tach i got didnt work, i guess the dp stuff aint so great. so i replaced it with a ebay tach that was the same size. that worked for me. also check out the thread about replacement tachs here in this section, it has other great ideas.
a pic of mine, (only taken cuz i hit 25k woohoo)