I have had a PC/rc Products onboard temp gauge for the last 8 years in my old RC10GT, and it has worked great. you HAVE to drill a mall hole in the head (easy on the side of a os .12CV), and screw in the sensor. Gives temps in Celsius or Farenheit, and constant temp or max temp settings. Cost was about $35 bucks 8 years ago...

I don't know if they are still in the market, though. Good, tough unit. I wash it with a toothbrush and soap, the tap water @ the sink to rinse it, and that's it. Just make sure you dry it out before powering up.

Hope this helps.