yeah you could put a tach on it. I had a race kart 100cc yamaha 2 stroke with a digital tach. I think it was just a ring around the spark plug and yeah temp sensors can also be a ring around the spark plug.
i used to race karts, we had one tag 125cc and two kt100cc karts and we had these on all three of them. it is all digital but they have a tach sensor which just zip ties up against the spark plug wire and it senses current i think and it shows rpm's. you can also get a cylinder head temp sensor which just goes under the spark plug and you can get weld in sensors for exhaust gas temp. its expensive but if your gonna spend money on a guage this is what i'd get. Mychron 4 2 Temputure Kart Racing Gauge: eBay Motors (item 120591746396 end time Jul-11-10 20:00:00 PDT)