I've been using a Tiny Tach reliably since sometime in the eighties on a variety of engines. The battery is only supposed to be good for five years. You have to be careful about other people mashing the reset button though if you care about the hour meter function.
Leading Edge Airfoils has some really nice 2" round gauges that aren't too pricey. There is a search function in their online catalog to get past all the Rotax parts. They also sell Tiny Tachs, sometimes cheaper than Tiny Tach does.