Yes -- you can add a warning light to the existing alarm system. Go to any auto store or Radio Shack and buy a 12 volt pilot light of whatever color you prefer. Make sure the light is 12 volt. If you go to Radio Shack they also have an assortment of 120 vac pilot lights but they won't work in your application.
1) Run a 12 volt POSITIVE feed (hot when the key is on) to one terminal on the pilot light.
2) Connect the other terminal on the pilot light to the ground wire from the temperature and oil senders.
3) The two senders I just mentioned apply ground to the alarm system which sounds the alarm horn. With the light connected to that system, should an overheat or oil level alarm sound, the light will also light. Just make sure you make the connections to the GROUND side of the alarm system -- not the 12 volt side.
Note that this is not exactly like the "Check Engine" function used on modern computer controlled engines but it does what you want for just a couple of bucks.
